The eleventh-hour delays for the PC versions of Ubisoft's games have become standard ops. If the delays are part of some piracy prophylactic, it functions as another layer to the company's "successful" DRM. The last minute Ubisoft PC delay tactic has been used most recently on From Dust and Call of Juarez: The Cartel, and goes back further with titles from the Assassin's Creed series and Splinter Cell: Conviction.
Update: Ubisoft has confirmed the PC version of Driver: San Francisco has a North American street date of September 27. The Xbox 360 and PS3 versions will launch September 6 in North America.