Since to falsely accuse a woman of illicit sex (fornication or adultery) is such a serious crime and grave sin in Islam, as indicated in the above verses [see Quran 24:4] it is quite clear that to force a woman to commit such an act must be a much more heinous crime, deserving of a much greater penalty. If to tarnish a woman's honour and character by word of mouth is regarded so seriously by the Quran then obviously it is much more serious to assault her bodily and coerce her by physical force to commit that very same act a mere accusation of which destroys her honour.
Exactly what the punishment for it should be, would be determined by the law of a Muslim country (assuming that it is a law sanely derived from the Quran and true Sunna), and then applied in different cases according to the seriousness of the offence.
There are two Hadith reports in Tirmidhi (under Abwab-ul-hudood) regarding incidents of rape. In these reports the woman was exonerated and the man was punished. (The reports mention the man being awarded the "stoning to death" penalty, but that may be because the incidents occurred before the revelation of the "flogging" punishment.)
According to the second of these reports, a woman was on her way to prayer when a man came upon her and raped her and went away. Some people passed by her and she reported it to them. They went and caught a man and she identified him. They took him to the Holy Prophet. When the Holy Prophet sentenced him to stoning, another man confessed to the crime. So the first man was let go, the woman was told that she was forgiven by Allah (i.e. there was no blame attaching to her) and the one who confessed was ordered to be stoned.
It can also be seen that there were no eye-witnesses in this case, and therefore other evidence (e.g. the accuser's report corroborated by other circumstances) is sufficient to convict the accused.
Taken from: http://lahoreahmadiyyamessageboard.yuku.com:80/forums/2/Holy-Quran-Study