Mine were all Evo to Evo calls. Wait til you do a video call with someone on an Evo, you will get the feedback on Fring, I guarantee it.
It doesn't matter if you are using a headset or not because the other party would get the feedback, and you would hear it.
If you are both using a headset, it might fix the problem. But even when the company first demoed Fring in that video, they were getting the feedback too.
Trust me, I've probably done hundreds of video calls since I got my Evo, all Evo to Evo, and there is an annoying audio feedback on Fring.
Now I haven't tried it with a wired headset, so if both parties have one it will probably correct it. I suspect this is why Qik requires one party to hit a button to talk.