Having talked at length both here and over at Geekplanet about the DSK arrest and the subsequent collapse of the case, and having done so entirely from the position that there was a horrible undercurrent of sexism and/or classism (if that's a word) about a lot of the so-called "defences" offered up for the man, I should note that one can go too far in the other direction.
To be clear, Hadley Freeman certainly has more than one very good point here. In terms of public opinion, there was a marked tendency to defend DSK by attacking Diallo, and whilst that might not necessarily be out of bounds per se, the nature of the attacks was frequently pretty awful. Moreover, Freeman is entirely correct to point out that anyone without expert experience who tries to argue that a woman is behaving in a way that is incompatible with the trauma suffered after a sexual abuse is a dangerous idiot.
But all of that is a separate issue to the fact that, as Freeman mentions, the Manhattan DA decided to drop the case. The court of public opinion has reacted disgracefully in this situation. The legal system, so far as I can tell, has worked exactly as it's supposed to. Freeman's arguments that all the questions over the plaintiff's honesty and reliability can be reasonably explained is all very well (though I wish she'd given a link to Diallo’s lawyer's argument about the phone call to the friend, as that seemed pretty important at the time and this is the first time I've heard it's been debunked), but the legal system demands more than that. Freeman pours scorn on the idea that Diallo's history of lying should be considered more important than the nature of her vaginal bruising, whilst skipping over the fact that Diallo's injuries could be ten times more important than what she said to immigration officials, and it still wouldn't guarantee a conviction.
Besides, at the risk of being blunt, suggesting that being prepared to lie to authorities about being raped to gain access to the country should be considered entirely irrelevant to whether someone might lie to authorities about being raped to gain money seems to be an act of wilful intellectual blindness. Moreover, whilst "she isn't behaving like a rape victim" is a much more distasteful stance than "she isn't behaving like a competent blackmailer", I'm not sure it's a great deal less convincing as an argument - I'm pretty sure there's lots of people out there who commit crimes without realising just how awful they are at it. Hell, raping a woman in a room under your own name whilst in a foreign country and whilst knowing any hint of scandal would cost you your job and your chance to become President sounds like an amazingly stupid idea, but Freeman is perfectly happy to believe that that's what happened.
The problem with Freeman's attitude to the DA is that there are three possibilities here, not two. It is not just about whether or not it could be proven whether DSK raped Diallo. It is whether a) it could be proven that DSK is lying b) it could be proven that Diallo is lying, c) that nothing can be proved. Everything Freeman says about b) being far more difficult than many have claimed is true, or at least arguable. But all that does is push us further into the massive chasm that is case c), a gap, of course, that is so large entirely deliberately.
There is a significant and ugly attitude problem regarding rape cases and those who accuse others of rape, I'm not denying it. Nor am I unaware of the truth in Freeman's words about the despicable but pernicious idea that only certain kinds of woman can be considered credible when they claim to have been raped. But the hard truth of the matter is that it doesn't take much to render a guilty verdict impossible when the only witness to an alleged crime turns out to be willing to lie to authorities when it suits them.
Indeed, turn the situation around. Suppose DSK accused Diallo of seducing him before threatening him with a rape accusation should he not pony up a few hundred grand to buy her silence. She, in return, argues he offered her said amount of money in exchange for sex, but refused to pay up post-coitus. Diallo is arrested on suspicion of extortion, but it then turns out that years ago, DSK lied to American authorities about having slept with a prostitute.
Our only witness, who is also the plaintiff, has been proven to lie to the authorities to get what they want. Moreover, they’ve made (roughly) the same claim both times. Would Freeman still be arguing that dropping the trial is anything but the right call given the chances of conviction? That this decision represents a larger problem regarding how often rich white men are believed when they accuse others of extortion? Even if there is a single piece of evidence consistent with DSK's story? 
Obviously, I don't know whether Freeman would still be so convinced the system had failed if this was the case. Furthermore, I don't know whether or not the forensic evidence is such a slam-dunk that the "credibility" issue shouldn't make any difference here (and if US law works like UK law here, the prosecution couldn’t keep Diallo from the stand if the defence demanded she give her testimony). Neither does Freeman, however, judging by the link/argument she offers. Again, someone appointing themselves a legal authority should not be taken any more seriously than those appointing themselves a psychological authority.
I realise this is quite a long post considering Freeman's article is fairly short and a lot of it is quite correct. Moreover, whilst I think she's conflated ugly public sentiment with ugly behaviour from the Manhattan DA, it's possible she's not trying to argue against dropping the case at all (the article seems a little nebulous in that respect). I just get nervous around the idea that something has gone wrong when the tremendously exacting burden of proof both we and the Americans require to gain a conviction doesn't include a known liar with one piece of consistent evidence.
P.S. I think this fits into a larger issue, actually, which is how horribly difficult it is to successfully convict someone of rape. The two main reasons most commonly cited for this are firstly that the nature of the crime is that there are almost never any witnesses, and secondly that it's far too easy for a perpetrator to argue that sex was consensual. Freeman is right to be angry about how willing people were to get ugly in their willingness to buy DSK's claims, but that doesn't have any bearing on the first problem: a single unreliable witness. Freeman basically seems to be arguing that because it's too easy and too common to label women as liars, the defence should have to prove that actual liars are clearly lying in this particular case. Which is one way of increasing the rape conviction rate, I suppose.
 There are of course flaws to this analogy. Probably the largest one is the fact that after his arrest at least one woman went public with similar accusations against him. I've left that out of consideration mainly because the rules over what is and isn't allowed as evidence in terms of other accusations are a bit complicated, and I don't have any understanding of them. I do know that you have to be very careful even in mentioning a defendant's previous convictions, let alone accusations of previous crimes. Maybe that does tilt the scales a little, though - I admit I don't know.