Fring is one of the more interesting mobile VoIP applications I've seen. It manages to use the regular IM capabilities of a number of IM networks-including Skype. It also uses their voice capabilities-yes, even Skype. And-get this-it's all data, i.e. even the VoIP leg doesn't travel over the PSTN network to your phone.

Several months ago, I did a couple of test calls with my Nokia E70 using AT&T's EDGE network-way before they upgraded the speed on it-and it actually worked okay for Skype calls. However, you had to have a pretty good EDGE connection, else it wouldn't work. Of course it worked okay over WiFi.

The folks at Fring have improved their client on the Nokia devices to support seamless roaming between WiFi and your cellular network. If while running Fring, you happen to come across a WiFi network you've seen before (and are configured to use), Fring will automatically switch to it. When you move away from the WiFi, Fring will switch back to your cellular network.

I wish all my mobile apps did this. Automatic switchover between networks, that is.