Maybe they're delaying the game to incorporate innovative features such as in-game rewind and the ability to share custom designs!
Well I don't think Forza is the first game to have rewind, and I really doubt Kazunori Yamauchi wants the online to be all about decal trading . . .
Considering Gran Turismo is Sony's biggest gaming IP, I think the last thing they want to do is rush the development and release of the title. I keep seeing people saying it's been 5 years etc, but the Ps3 turned 3 years old like 2 months ago. So GT5 comes out in the Ps3s third year on the market.
And let's not forget; While Forza 3 performed well with critics, it was a complete disappointment at retail- specifically in North America. If you were a Sony exec and you saw Forza 3s numbers, you wouldn't have any incentive to rush the release of what may very well end up being the highest selling Ps3 game this generation.