Hey just to let you guys know, and its probably been posted before but VOIP works with v3 firmware for the E61. I have just set it up and using UMTS/GPRS rather than WiFi works. The call quality is excellent. Only thing is that over UMTS/GPRS there is about 1 to 2 seconds delay but overall the quality is great from the E61 to a landline number.
I did get this to work with v1 firmware but the quality is poor and barely able to converse with the other person on a landline.