Update: The instructions here are no longer valid, because Google has since closed off inbound SIP calls into Google Voice. However, I have posted a new solution for making free phone calls with Google Voice, Gizmo, and Asterisk.
I recently switched my GrandCentral account over to Google Voice. I fooled around with Google Voice quite a bit and was quite impressed by the many features. It took quite a bit of work, but I finally figured out how to set up Asterisk along with Gizmo to use Google Voice to make free telephone calls to anywhere in the USA.