"We're still working on our review, but the makers of Google Voice app OneDialer took to our forums to announce the latest version of the app. Currently for Windows Mobile 6 Professional (touchscreen devices) only, here's what you get: SSL encrypted communications, It's fast, SMS and voicemail notifications, [and] Native dialer interception."
It's Touchscreen only, as of now, and is $7 for a single user license, but if you really want to use your Google Voice number in Windows Mobile, it's now an option!