March 2010 wouldn't be surprising. If it was hitting in 2009, it would've had a date at the end of the trailer shown at the VGAs. Uncharted 2's trailer, if you recall, ended with the words "Fall 2009".
The only question is whether GT5 will hit in 2009 or not. On the one hand, Yamauchi is a perfectionist and Sony basically lets him do what he wants, so a delay to 2010 is possible, but on the other hand, Gran Turismo is SCE's biggest franchise, and would do wonders for them over the holiday season. That combined with the fact that the game has been in development for quite some time (plus Sony's financial troubles) could lead to them forcing Yamauchi to release the game in 2009.
I expect one to be released in 2009, however. Sony usually releases 3-4 big titles over the holidays. In 2007 it was Uncharted, Ratchet & Clank, Heavenly Sword, and Warhawk. In 2008 it was LittleBigPlanet, Resistance 2, MotorStorm 2, and Socom: Confrontation. In 2009 it'll be Uncharted 2 , Ratchet & Clank Future 2, and...? I thought God of War would fit in perfectly with the others, but that's apparently not going to happen. I think GT5 would destroy the sales of everything else around it if it hit over the holidays, though. That'll probably sell a few million the first week alone.
I also can't see both hitting in Q1 2010, as both games are their only remaining system sellers (games with a hardcore following that may have not yet purchased a ps3), and I can't see either hitting any later than Q1 2010. I think it's either/or for both. One over the Holidays, and one in Q1 2010.
