In order to change the phones Google Voice will ring to, you need to authenticate that phone with the system (entering in a 2-digit code.) I think this is to make the service useful, as opposed to annoying by allowing arbitrary dialing and receiving of telephone calls (phone phreaking).
Here is a screen shot of how I can change the phones Google voice will ring on:
When I am at work, I just forward all my calls to my work number. Once I get off work, I have them forwarded to my cell phone. It's really easy and convenient.
I hope to post some additional screen shots to my blog showing how cool this service is. If anyone has any specifics they want to know, just send me a comment. Hopefully this isn't violating the terms of the beta, but if it is, google should "do no evil" by gently reminding me before pulling the plug!
I just wanted to throw this post up really fast so people could try calling me with the embedded button.