Microsoft's primary strategy for building it's exclusive library is to buy exclusives from third parties, in contrast with Sony, who prefers to make their own internally. However, third party exclusivity contracts never call for permanent exclusivity, instead calling for exclusivity for a certain period of time (usually 6 months, sometimes a year), and in this age of high development costs, this results in many 360 exclusives eventually going multiplat, like Bioshock and Eternal Sonata, rather than just staying exclusive, like many games last gen (though even last gen some of the bigger ones went multiplat, like GTA and MGS2). For other games, MS paid for timed exclusivity for a very short period of time (one month for Rock Band 2, a few months for the original Rock Band in Europe last year).
The only 360 games that are guaranteed to not come to the ps3 at some point are those published by MS, which aren't all that many (particularly now that BioWare and Bizarre have been picked up by larger publishers, and Team Ninja is now defunct, though MS only published NGII for them).
Luckily for ps3 owners, Sony now has a rule saying that developers can't release a game on the ps3 after the 360 version has released without adding more to the game, so the ps3 versions of these games come with quite a bit of extra content versus the 360 originals.
Come to think of it, I wonder what this rule means for the short timed exclusive Rock Band 2, or the delayed ps3 version of the Last Remnant?
In the end, I don't mind all this timed exclusivity business. I'll play the ps3's exclusives now, then play the 360's exclusives when they head to the ps3 or PC down the line. ;)







