Through lucky coincidence and my geek research habits, I am one of the minority who can get the most value out of Google Voice. Google Voice + Gizmo + [sip client] is a way to get a phone number and make outbound calls, inbound calls, and SMS from your PC for free. This is the functional equivilant of getting SkypeIn/SkypeOut/SkypeSMS for free instead of paying $80+/yr for it. And I think it's a better solution than the paid Skype product. And it's much better than paying for a landline.
A friend saw my setup and wanted to duplicate it. My reply was lengthy and oversimplified to the point of inaccuracy, but it might be a springboard for your own cheap VOIP ideas.
I don't think [routing Google Voice to your PC] is directly do-able at the moment, unless one already has a Gizmo/sipphone account.
* gizmo was a particular SIP provider. SIP is the main open source VOIP protocol, and is what the mighty Asterisk PBX servers use.
So far I have used the Ekiga and Twinkle SIP clients here on my box. I bounce between them.
* you can think of SIP as similar to Skype's functionality, only open source. It's not really accurate, but will help get the head wrapped around it. You do the same kinds of things with a free SIP client as you would do with Skype, except there are no easy SIP-Skype gateways. Skype definitely has the "mindshare".
* Google bought sipphone/grandcentral/gizmo.
* gizmo registrations are suspended, but they do have a "notify me" form for when it reopens
* Google voice DOES allow setting up a SIP account as a phone
* BUT it currently only allows Gizmo accounts
* until Google either reopens Gizmo registrations or allows non-gizmo SIP routing "the game is locked and nobody else can play."
I think if/when Google allows non-Gizmo SIP it will be a game-changer.