OctroTalk has released version 1.0.5 for Windows Phone. You can now send instant messages and make Voice Calls to GoogleTalk/Gmail users.

In the voice call feature, even though we did our best, there are
still some known issues -
1) We cannot play the sound through the headset. There is no API to
route audio through there. Ask Microsoft for help .
2) There is about a 1-2 second delay in playing remote voice. Again, a
problem due to the APIs.
3) Echo - since we can't use the headset, and there is no echo cancellation API.
4) Incoming calls don't ring if your application is in the background.

The new version can be downloaded from the Windows Phone Marketplace at -
http://windowsphone.com/s?appid=42f3...4-e290bbd54cde