I've played around with the SIP functionality a bit using my home Asterisk server. Over WIFI, call quality was excellent. The only annoyance was a bug with DTMF wasn't being sent during the call. So I could not access my voice mail. According to my research that partcular bug is fixed and will be in the first firmware update.

Using the internal data connection was more dicey. I've only got EDGE server where I live. There was a lot of drops outs and my voice mail to myself was cut short due to the drop outs. So probably no VoIP for me until T-Mobile rolls out their 3G service locally.

There is an application called DialCentral which has Google Voice application functionality. So everyone should have no problem using Google Voice on the N900. With the use of Teleparth framework we have the potential for some great integation. Google Voice could be added and integrated into the N900. The possibilties are amazing. I don't think any Smartphone on the market would do such a thing. Having Google Voice as one of the call types under the Phone application. Having SMS message from Google Voice showing up in Conversations and having the notifier server letting you know hen your have new GV voicemail and SMS messages. Absolutely mind boggling.