Google Voice app has finally been allowed by Apple app store. For Microsoft, the Windows Phone 7 app store already is accepting Google Voice, where an unofficial client has already appeared. The app called "GoVoice" will provide Google Voice functionality for WinP7.
Another mobile app developer, Sean Kovacs, of GV Mobile+ app has plans to incorporate Google Voice on WP7 as well. GV Mobile+ already has a large amount of features, mostly pulling from Voicemail and SMS abilities. On the iPhone, it also allows you to transfer your contacts over from Google Voice to iPhone, so for the WinP7 version, we should expect very similar features. VoIP still has yet to be added in this app, although Kovacs has said he's working on it.