Of course it won't be an exclusive (although I bet they make a deal for some exclusive DLC). But getting Rockstar to deliver their big 2012 title at launch serves a number of purposes:
- sends a signal to other third party developers/publishers that they're serious about pushing games other than epileptic flailathons on the console
- shows "core" gamers that their brand new system won't just be a rack for grandma's pantaloons in six months
- provides a bit of a showcase for their new controller screen ("Your cousin will whine about taking him bowling RIGHT ON THE CONTROLLER")
It's all win for them, with no drawbacks. GTAV, a few more PS3/360 portalongs, plus whatever twee games they'll have ready, best of both worlds for them.
Too bad about the online, though - you just know it'll be worse than PSN circa PS3 launch.