Hi, yes, it's possible. This is what you're going to need:
-Google voice account (obviously).
-Gizmo project account
-Fring (other SIP software should work fine)
1. Sign up for a Gizmo account:
http://www.gizmoproject.com
Install the Gizmo software ON YOUR PC. You need that for the verification call from Google voice).
2. In your "My" page on the Gizmo web site, you see "SIP number" under "Account overview", copy that number.
3. In your Google voice account, go to "settings" > "Phones" > "Add a new phone". Name it what you want, put the SIP number from Gizmo and choose "Gizmo" as the phone type".
4. Install the Gizmo software on your PC, login. Make the google verification call. Gizmo rings, pick up, it asks for a verification code, just type it on your keypad. Your Gizmo number is verified.
5. Go to fring.com and download fring for your PocketPC.
6. When it's time to add an account, double click SIP, then choose gizmo. Enter your username and password, and you should be able to receive google voice calls via wifi as long as fring is loaded.
You can uninstall Gizmo from your PC if you want.
The big problem of course is that this will eat your battery alive :-/