It is possible to implement getUserMedia on IOS as a bolt-on to Apple's WebKit engine.
A proof-of-concept WebRTC-enabled browser caller 'Bowser' was released last year by Ericsson Labs but the current release on the App Store is missing nearly all the functions we come to expect of a browser. Furthermore I have experienced a string of problems with Javascript control of the <video> element once bound to a local WebRTC stream.
I hope the Google team will go down this route for Chrome, if not just to twist Apple's arm into supporting getUserMedia quicker than they would otherwise.
I've been waiting over a year since Opera's getUserMedia-enabled browser hit Android for anyone to catch up. In the mean time Opera have let their lead slip by introducing a slew of bugs; it's only now we're starting to see Chrome on Android catch up - with a promise of Firefox to follow. I can't see Apple will be able to delay much longer when G+ and/or Facebook start offering browser-based full video calling!