The folks at 2K Games passed along via their Mafia II official page
that the demo will be available on Tuesday, August 10th. As they are usually so kind, they have also provided some time windows when the demo will be up on PSN, Xbox Live and the Mafia II website:
MAFIA II DEMO GOES LIVE TOMORROW
The Mafia II demo goes live tomorrow for the PS3, Xbox 360, and PC. Want to know exactly when to expect the demo so you can be the first to download it? We have time windows so you can plan accordingly!
PS3 North America: August 10, 4 - 6 PM PT / August 11, 12 AM – 2 AM GMT
PS3 International: August 11, 12:01 - 6 AM PT / August 11, 8 AM - 2 PM GMT
Xbox 360: August 10, 2 - 4 AM PT / August 10, 10 AM – 12 PM GMT
PC: August 10, 12 AM PT / August 10, 8 AM GMT
Just a hunch on my part, but I am guessing that a solid half of the gamers may be busy doing something else tomorrow, so getting the demo downloaded might not require much of a wait.