Now that I have my N900, I'm running into this problem, too. I always get the SIP call in first, and the 3G call shows as "missed call".

There's nothing wrong with this as long as I'm home (Wifi) and plan to stay there. But it's not always ideal when you're out and your internet-/voip connection is 3G... or you change between access points. I need to manually connect/disconnect from my sip account, depending on where I am. Of course I forget.

Is there a solution that works right now or do we need to brainstorm?