I created a Service to send a selected phone number to Google Voice to be dialed. To do this, I actually use two files: The service, which should be installed in your user's Library/Services folder, and the google-voice-dialer php script, which does most of the heavy lifting; install that wherever.
Download the zip file (78KB), expand it and open the resulting folder, move the Service to your user's Services folder and the php script to wherever you'd like it to reside. Before using this Service, you need to open it in Automator for some editing. Within the AppleScript code in the Run AppleScript Action on the right, scroll through the code and change the following properties: usrnm, psswrd, phnm, myFilePath, and myFileName. Save those changes, and you should be good to go.
To use, select a phone number in any Cocoa application (Safari, Mail, Address Book, etc), Control-click on the selection, then go to Services » gvDialer, and Google Voice will dial the number.
[robg adds: I haven't tested this one.]

