The handover isn't done in the middle of a call as far as I know, so if that's what you're looking for... sorry, I doubt it's there. What I can tell you is that you can tell the N900 that you want it to connect to wifi when it's available. When wifi is unavailable, it switches over to GSM. Like previous tablets it will search every so often while you are on GSM to see if a wifi access point you've defined is around. You can change the search time to conserve battery life if you so choose. The default is 10 minutes just like the previous tablets.

Basically, when you get home or leave home, you can pretty much rest assured that you're on the proper network and won't be left without a connection for very long, if at all.