Would any of this be surprising? The last three PGR games were all released two years apart (2001, 2003, and 2005) -- thus, by induction, 2007 would be the release date for the third sequel. Also, developer Bizarre Creations has worked exclusively with Microsoft for the last four years, and the entire PGR series has been Xbox-exclusive.
This "leak" is probably nothing more than an educated guess. The existence of Project Gotham Racing 4, a sequel to three critically-acclaimed racing games, is about as surprising as the existence of gravity (hey, it's only a theory). A 2007 / early 2008 release would put Bizarre right on track. However, don't hold the developer accountable if they miss a deadline set by an unrelated, outside party.
In other news, sources tell us PGR4 will include some fast cars and "race tracks." It is unconfirmed whether or not J Allard will make an appearance.
A look at 7-11's potential Wii slip-up
[via Euro Gamer]