Far from being an expert, I suspect V8 was designed as being pluggable but that the integration with Mozilla's UI platform (XUL and Gecko) will be problematic. Mozilla's code is too old for such big design changes. I'm willing to be proved wrong though.

In any case, IE aside, the competition between Mozilla (Gecko + Tracemonkey) and Chrome (Webkit + V8) is what will drive the web forward. Competition is always good for the consumer.