Remember back when people said BC fights wouldn't be boxable because of all the movement and MC and shit, especially fights like Nazan, Kelidan the Breaker, etc.? Yeah. Not only did we box 'em, we boxed 'em with single-class teams and, in some cases, 4-manned it a few levels above. People said the same thing about Northrend instances and, while we haven't been as easily successful doing things like 5-shaman runs without overgearing, the basic trend held, even through the fights that people insisted would absolutely NEVER EVER EVER NEVER be boxable like Oculus and ToC. HA! I suspect it will be exactly the same with Cataclysm 5-mans, it will just require (as every other new experience did) a rethinking of how you are set up for the new fights.

Of course, there may be certain gimmick fights that wouldn't be doable but I've yet to see Blizzard implement anything like that and I have no reason to think they are desirous of screwing over multiboxers.

Consider as well that a mediocre multiboxer like me is about as coordinated as a really bad PuG, so Blizzard can't really do things that hamper us too much or they risk alienating a much larger portion of their playerbase.