Tried it again. N97 to N97 mini. It worked the first time I tried it and wouldn't work the next 10 times. The phone would ring and then give up or would give up immediately after I answered. After about 10 tries I did manage to connect but got disconnected after a second. I'm trying this with both phones on the same WiFi network.
The front facing cameras worked this time around though.
Update: It seems to work fine from iPhone 4 to Nokia but not Nokia to Nokia. The camera quality on the N97's front facing isn't that great compared to the iPhone 4.
It may be worth trying the two phones on different networks - it is possible that the Fring servers are confused when similar devices from the same IP address are trying to talk to each other...