You can't return to a non-wifi call, but you can shut down the video and continue the audio call on wifi by simply hitting the home button...
To resume video, do you just hit the home button again?
I'd assume (haven't used it) that you'd just go back into the phone app.
That or press the green bar flashing on the top. Just like switching to another app when you are in a call using a 3G or 3GS. Press the Home button, and you'll see the green bar flashing across the top, but the call is still active and you can continue your audio only conversation.. Press that green bar or the Phone app and your right back to the Phone screen. Or with the case of FaceTime back to your video call.
You press the flashing Green bar across the top of the screen to resume the Video call.