It won't be a shock to us, but it is official. Just saw this at Voodoo Extreme:
GTA4 Is PS2 Exclusive 2:37 PM - Andrew "Andy" Burnes - Consoles: Sony Playstation I/II
The existence of Grand Theft Auto 4 has been confirmed and Sony has announced that the title will debut exclusively on the Playstation II. The likelihood of an exclusivity being signed is high.
More details as they come in.
This doesn't mention a release (or projected) date, but it's good news for PS2 owners. It's still gonna be 6 months or so later for us PC gamers.