In spite of a misunderstanding on the internet (courtesy of an interview on Major Nelson's podcast), Grand Theft Auto 4 will have multi-player capability on all versions. So don't flip out.
"It's always something we've been open about, both versions will support online multiplayer capabilities," a Rockstar spokesperson told GamesIndustry.biz, although exact details on those capabilities have not been announced as yet.
Don't flip out. Seriously. But do check out the game's trailer, and remember why you care.
GamesIndustry.biz: Rockstar clarifies GTA IV speculation