Among the interesting details discussed (some of which have also been revealed elsewhere):
- Previously rumored 16-player online play is confirmed.
- The online maps aren't changed in any way from single-player -- you can still drive vehicles from one end of the city to the other, if you want.
- There's no co-op option for the single-player missions, but many online modes allow for cooperation.
- Ranked online matches earn players in-game cash for character customization. Higher difficulty means more cash.
- Race mode can be played with or without weapons.
- The volume of other characters voices will vary depending on their distance from you. "So if you hear an enemy's voice, he's probably right behind you and it's too late."
Note: GameTap's servers are getting hit pretty hard, so the Read link may not load for a while.
Update: They're all live! GTA4.net has posted links to numerous multiplayer previews. (Thanks Cal!)