CaptainABAB
Member
RT allows other browsers as much as iOS does. They'll have to use IE's rendering engine, but can wrap it in their own skin with new functionality and other gimmicks.
They do not HAVE to use the IE renderer. The only limitation is that the RT API does not allow dynamic compilation of code - which hampers JavaScript performance in Chrome/Firefox. Obviously this is a major downside, so the work around is to use IE as the renderer or do what Opera mobile does - render on a server and stream the results to the client.
This same limitation exists on iOS.