Since I work for an Xbox magazine for a living, I think I can shed some light on this one...well, not the GT5 part since that's a Sony game, but Forza 3...
While visiting the developer's offices a few months ago for a story on Forza 3, I did specifically ask Dan Greenawalt, the studio head, about putting the Tesla in the game. He told me it's not in there because tuning/user-created content is such a huge part of the game, and with the Tesla, of course, there's no ICE to tune!