There's a market app called Google Voice Dialer that will route your call through your data connection. Supposedly it's integrated into the dialer so you don't have to run the app to do it. It's $1.99. I haven't tried it myself.
http://gvdialer.cpedia.net/
I am trying to use google voice as my primary number, but having no mms bugs me. Also, not being able to send sms to multiple recipients could be a problem once in a while. Having access to all your texts and voicemail online is really cool though.