Unless you own the IP, is nigh impossible to bargain with a third party for full exclusivity.  They're simply not willing to completely give away the rights to a game like that, and if they were... they'd give the publisher/buyer the IP.  Best we can tell, Sony isn't even publishing.  In these cases exclusivity is always bought for a certain period of time.  After that time is over, the owner of the IP can do whatever the hell they want.

The only question is whether Rockstar will want to to port the game once the period of timed exclusivity has ended.  Given they were willing to port their titles to even the original Xbox, I don't doubt they'll be working on a 360 version of the Agent shortly after release, especially since the game is being built on a multiplatform engine.

That being said, this situation is totally unlike LA Noire, and using one as evidence of the other is silly.  Sony was initially funding and even publishing LA Noire, but the project was bleeding money so they dropped it.  Instead of making Rockstar pay all that cash back, word is they bargained to make Agent a ps3 exclusive instead.  It will at the very least be timed exclusive as a result, however I don't believe it'll be anything more than that.

Of course, Sony could announce at E3 that they're publishing the title, which would change everything.  The publisher has yet to be announced, afaik, though I assume it'll be Take Two.

This is a rumor with no evidence that I have yet seen to back it up.  All we know is that Sony effectively traded LA Noire for Agent.  I doubt either have anything to do with GTA going multiplat.

Microsoft holds the publishing rights to Ninja Gaiden II, yet a slight name change and some additional content allowed Tecmo to easily circumvent any contract they had with Microsoft and release Ninja Gaiden Sigma II on ps3.

The only way Microsoft or anybody else can guarantee permanent exclusivity is if they own the entire IP. 

Of course, sometimes studios have other reasons why they will likely never go multiplat, ala Epic and Gears of War.  Microsoft made the franchise what it is, and going behind Microsoft's back (thus abandonding their financial support and marketing muscle) would be foolish.  Making Mass Effect multiplat, on the other hand, would cause EA little harm.  The only question is whether they consider it worth it to port a game to ps3 over two years after the original and a year after the sequel have already been released.  Mass Effect would probably sell even worse on ps3 than the original Bioshock.  At this point it'd be a waste of time, imo.

Well Agent used the full power of the blu ray disc, unlike GTA IV. So which then it will be ps3 exclusive.

its always possible that it is a timed exclusive but eitherway ps3 gets it first, there is another factor that many people dont consider if Agent takes full advantage of the ps3 then its not a question of timed exclusivity it just wont be possible on 360, if they create a ps3 engine plus fill a bluray it definitely wont see the light on 360.

if its using the GTA engine i can bet its a timed exclusive

sony published heavy rain , so no it wont be coming out for other platforms.

but future titles from QD can go multiplat as they are technically a 3rd party but sony owns heavy rain ip

Neither LA Noir or Agent interests me in particular... will Agent go multiplat? Maybe, but even so I doubt that it can become much bigger then RDR... and lets be honest even if that game was exclusive for either platform, it still wouldent have been such a huge deal

ya your right they dont own the actual ip but they are the ones publishing their games so no way will even the next QD game be multiplat.

It will be a tragedy if Agent doesn't come to the PC. If the game is as epic as has been implied then it deserves to be run on the best hardware at a good framerate in 3D at full HD with all the settings turned on at 60FPS (120/2)

Yep its a swiss army knife statement.