WnN2 - Thank you. 
Mohamed_GANgsta - Not a dis, just lay off the over sized text. 
Granted this only speculation, but if R* has a plan w/ the exclusive 360 DLC, it's probably to get the PS3 owners to want it that much more.  For all we know, Sony wants to hold off so they can charge a premium if/when they finally do get DLC. 
R* could've also been looking at  Sony's initial release of the PS3 as a shortage or something.  Remember people camping out at Wallmart  & the insane prices they sold for on e-bay?  
Previous titles had exclusive release but eventually released to other platforms.  
As I said, only speculation.