Who did Nigel Short really play on the ICC in 2001 when he thought he was playing Fischer? In 2001, Nigel Short went public with the opinion that he had in fact played Bobby Fischer on the Internet Chess Club.
This was denied by Fischer himself, and the consensus seems to be that it was not in fact Fischer that he was playing. However, though I have the impression of reading something about who Short was really playing some time ago, I cannot remember this for sure and now cannot find a conclusive source on who Short's opponent actually was. Has there ever been a definitive statement on this?
Nobody knows who he played, because nobody has come forward to take credit; however, some chess sleuths compared the moves against various engines and found that there was considerable evidence to support a particular engine (I believe it was called "blitz tiger") as the mysterious Fischer.  Because of the incident where Short chatted with Fischer, there must have been a human behind the account as well (many accounts on the ICC are run entirely by computers without any human interaction at all).  The ICC can probably do some user profiling to figure out who the player is either by comparing the location/IP/e-mail to another user or by doing some internet forensics based on information they gathered from the incident.  However, it seems like they either don't care or if they did find out, they don't want to share (based on what xaisoft said) so it will probably always be a mystery.  And, to be honest, it's a better story that way, and because all this is is a good story, that's a good thing.