Always include:

Device:Sony Ericsson
Carrier:AT&T
Country / Language:US
OS / Browser / build number (if applicable):android 2.1

I've set making call "use google voice to make all calls" in the settings.  When I make a call, it briefly shows GV dialing screen for less than 1 second and promptly switches back to my cell dialing screen.  The call still goes through my cell phone instead of GV app.