So it's more or less call forwarding on steroids. I don't see anything groundbreaking that I can't do using the call forwarding on my AT&T plan, with the exception that GV's more automatic and Googley (and will, though not yet, make the number you appear to be calling from your GV number)
The way it works is you use a web interface to enter the number you wish to dial and the number you wish to dial from. Then your phone rings, you pick up, and the number you are calling subsequently rings and the connection is made.
Incidentally there is a 3rd party app for the android that transparently handles all this for you-- you just enter a number to dial and it connects you with GV.