I thought Gamestop would take your money for pretty much anything. Even if they dont have it in the computer yet, they can put it aside as something generic right?
The Gamestop I go to in Ann Arbor, MI has a "Call list" for the PS3.
Basically you put down your name and number and they call you as soon as they can start taking PS3 reservations.
I think that's the closest we've got to reserving a PS3 right now...