Tough to say who this benefits.
On one hand, MS now has an extremely solid holiday lineup, littered with exclusive titles, most AAA but can't play the "Halo 3 and GTA4: only on 360" card this holiday season. However they can play it next year. Also, apparently the first excuslive episodes will still be ready by the time of release (or thereabouts) so the 360 version will be superior almost immediately, rather than 6 months later (all else being equal of course). Those who are going to buy a PS3 anyway, now have one less reason to buy one this year, and may now wait for further price relief or switch to 360.
Ont he other, Sony now has a perfect storm of their own next year with the release of MGS and GTA4 and can play their own version of "MGS and GTA: only on PS3" 2Q of next year. They may be able to have another relevant price drop before then so the disparity of between the cost for entry for GTA on both platforms might not be as large as it would have this year.
Personally i do think its a bit worse for PS3 since they just need to start stockpiling some reasons for the Sony faithful to jump on board. Going into this holiday without GTA, even multiplatform, hurts them because it keeps existing PS2 owners on the sidelines. Without this, there isnt a lot of familiar content to bring those existing owners over.