Answers to some of the incorrect information flying around this thread:

1. He didn't know she was 13.

Yes, he did. Polanski asked the girl's mother if he could shoot pictures of her for Vogue. There were two photo sessions (the attack took place at the second one). Polanski did not randomly bump into her. He knew her and her family, met her mother on several occasions, gained her legal consent for the photo shoot (which would involve full disclosure of her age) and he well knew how old she was.

EDIT: Thanks to Legolas's link:

http://www.dailymail.co.uk/femail/article-477609/The-dark-secrets-Roman-Polanksi.html

We also know that the Vogue photo-shoot was specifically going to be of young girls in the victim's age range.

2. He was never proven guilty.

Yes, he was. He did not flee the United States before the trial, he fled it after admitting his guilt.

3. It wasn't rape.

Yes, it was. Sex with a minor does not use the word 'rape' in its title, but since a minor cannot give legal consent to sexual intercourse, it is rape. The difference is a legally semantic one.

In addition, whilst Polanski's legal team has only ever focused on sentencing regarding the charge he admitted guilt to, the Los Angeles County Court has ruled that ALL of the original charges, including the drugging and indeed the possible use of the 'statutory rape' charge could be back on the table should he return to the United States:

http://www.nytimes.com/2009/01/06/movies/06polanski.html?pagewanted=2&_r=2&sq=polanski%20dalton&st=cse&scp=4

4. He's hasn't repeated the offence again in the past 32 years.

Yes, he has. He began a relationship with actress Nastassja Kinski when she was 15 years old. This is (just) the legal age of consent in France, but is not the age of consent in the USA or UK. His prediliction for young girls remained intact after the incident, showing that the experience did not rehabilitate him.

EDIT: Thanks to Legolas, I realised this incident happened in 1976, a year before the original case took place. There is no evidence that Polanski has re-offended on this score since the original incident, merely that when it took place it was not the first time it happened.

5. The victim doesn't want the matter pursued.

The victim did want the matter pursued quite vigorously until she settled a civil case with Polanski a few years ago where she was paid money to drop the matter. Now she wants the matter dropped which, whilst a point certainly worth invoking, is ultimately overriden by Polanski's flight from justice and also the debt he owes to society. Part of the punishment for a crime is ensuring that others are inspired not to commit the same crime. The high profile nature of this case has given rise to the belief that it is possible to commit a crime in the United States and then flee to France where you will not be extradited back to the USA. Also, given that the overwhelming majority of rape cases are still not reported, showing that the perpetrator of such a crime can still be brought to justice 30 years on is a powerful display of the law working as intended.

In short, the victim doesn't want the matter pursued as she has been paid off. Good for her, but that isn't the only issue involved here.

Clarification: The victim began civil proceedings against Polanski through the French court system in 1990. She pursued the case for seven years until they reached an out-of-court settlement in 1997, where she was given $140,000 (source: Legolas' link to the Daily Mail story). Immediately after the settlement was reached, she publicly forgave Polanski, whilst still maintaining he had 'drugged and raped her as a starstruck kid'. It is possible that this declaration was part of the terms of the settlement, although this is not confirmed.

6. The judge was planning to shaft him.

Possibly true. In fact, the judge was later taken off the case because of concerns over his competence. However, there are many legal recourses that could have been taken against the judge if he indeed reneged on the plea bargain. Since thousands of cases in California alone are dependant on plea-bargaining being permitted to get any kind of verdict, dishonouring one would have been a horrendous precedent to set. It is also unclear if the judge actually was going to do this, or was blowing off steam in a private conversation after being shown pictures of Polanski partying in Europe before sentencing.

I think that about covers it: he knew how old she was, he admitted that he did it, it was not the first time he had done it and the original victim was paid off not to pursue the matter any further, which has zero bearing on the legal side of things.