On the PS3 side, God of War 3(you know it's coming), White Knight Story, Ratchet and Clank Future: Tools of Destruction, Drake's Fortune(new Naughty Dog title), the future title from Fumito Ueda(behind Ico/Shadow of the Colossus), Lair, Heavenly Sword, Killzone 2(I enjoyed the first one, so sue me), Ninja Gaiden Sigma(not sure if this one will be ported), and Resistance 2(just titles already announced or hinted at, hopin for Suikoden VI to be announced as well)
On the 360 side there's Blue Dragon, Lost Odyssey, Cry On(another Mistwalker title) Infinite Undiscovery, Eternal Sonata, Halo 3, Ninja Gaiden 2, Fable 2, and Project Sylpheed. Mind you these are just games announced for the most part so far, there will undoubtedly be more in the future.
Multiplatform games I'd get are DMC4, RE5, GTA4, Assassin's Creed, Unreal Tournament 3, and Mercenaries 2, not sure if I'd get for PS3 or 360 yet, depends if Sony releases a rumble compatible Sixxaxis(Immersion and Sony did make up, so who knows). The current one feels too light to me.
If MGS4 and FFXIII are made multiplatform I'd get for whichever system I'd want to play them on.
And if they don't become multiplatforms then they'd just become 2 more reasons for me to get a PS3.