Naxx was the last boxable raid instance, and even then only a few people really multiboxed it while it was relevant content (until ulduar release). Some of the fights in ulduar are boxable but most really aren't well suited to it.

Imo there really isn't any reason to multibox raiding anymore, the impetus during BC times was that there were only a few guilds that could do the newest raid instances per server, and so there was a "gap" between those who could do kara/gruul and those who could do BT/Sunwell. You were in one camp or another. Boxing made kara easier and you could spend badges on gear.

Now however, Blizzard has split raiding into 4 pieces - 10 vs 25, and hard (10/25) vs regular (10/25). Right now ICC is a little hard past the first wing, but in a month or two it will be doable by virtually anyone. I've pugged up to putricide in 10 man. So theres no real reason not to simply pug or join a small time guild if you want to see icecrown, no point in multiboxing raids.

Where multiboxing IS nice is farming frost orbs (=> primordial saronite). It is awesome that I have 8 level 80s and I can run 2 heroics a day in < 45 mins and end up with 16 frost orbs, plus another 40 orbs a week from the weekly raid (running each char solo/dual boxed). Its gearing up my main insanely fast, he already has all the BS patterns and DPS plate items crafted, and I'm on my way to getting shadows edge made.

its also fun just playing alts in different ways, imo the future of PVE multiboxing is multi-class comps so you can multibox the daily heroic for gold/badges, and then solo play various classes for fun. its very different doing ICC10 as a melee DPS than as a caster or a tank or a healer.