Rocketpig, did you read the contract that MS has with R*? I know I haven't so it's hard for you or I to know what kind of smaller parts were in that contract for the DLC. And yes, MS did pay for the exclusive DLC, but it may have included a bit more in that deal that stated that demos/videos/etc that they released would show only the Xbox 360 version. Could I be wrong? Possibly, but I could also be right, we just don't know.
I'm not saying that it's decietful or wrong for them to do. GTA is a huge series and if they can convince people to buy a 360 instead of a PS3 to play it, it will pay off for MS and would be a smart move. If it backfires, it will hurt a bit.
And I expect both versions to look great and sell very well over the lifetime of the game.

Get your Portable ID!
My pokemon brings all the nerds to the yard. And they're like, "You wanna trade cards?" Damn right, I wanna trade cards. I'll trade this, but not my charizard.