http://lifehacker.com/5332309/use-ubiquity-to-make-google-voice-calls
The earth2marsh site posts a Ubiquity command that requires just a little bit of configuration to hook up with your Google Voice account. Once it's set up, however, making a call with your pre-determined default phone is a simple matter of highlighting a number, activating Ubiquity, and using the action verb "call."