Log into your Google Voice account (sorry, it's invite only right now)
Dial using the on-screen keypad
An alert pops up asking you if you'd like to make the call using a Google Voice access number.
It then dials and displays your Google Voice number as the outbound caller ID.
The current problem is that calls made from GV number aren't recognized by my friends, so they don't pick up the call. While a deal-breaker for most, this issue will soon be fixed when Google rolls out phone number portability. Personally, I can't wait.
I've got a pair of GV invites to give away to two randomly selected readers that follow me on Twitter and mention this post.