Minor feature update for Google Voice today, you can now use your own existing number for the service (as long as it is a mobile number). Using it this way doesn't give you all of the features that are available if you use a new number, but it does give a chunk. Primarily, 1-box voicemail with email transcription; no more need to check vmail on a cell and for a google number; it's all integrated.
More info here:
http://googleblog.blogspot.com/2009/...-existing.html
Also, I've got a few invites left so if anyone needs a google voice account, just PM me with your preferred email address and I'll shoot one your way.