According to Matt Grandstaff customers who have purchased digital editions of Fallout: New Vegas will be able enjoy their games at midnight day of launch:
Hey guys wanted to share some info on when the game will be available to unlock on Steam, Direct2Drive, and Gamestop digital download.
On the 19th:
12:01 AM Pacific Time (PST) for North America, Central America, and South America.
On the 22nd
12:01 AM Greenwich Mean Time (GMT) for territories other than North America, Central America, and South America”