Thanks for the great feedback everyone! Gonna go grab another copy at lunch and fire up the 3rd account when I get home tonight.

You will feel the temptation to do 5 though =P
That is so true and something that still bugs me a little. I'd hate to invest all the time getting these guys up to 80 only to find that they are never really going to cut it in instances. Did you guys who have went to 5-box do so for this reason? Or did you do it purely for the sake of 5-boxing?

Healer -> Still trying to find the best way to do this, so far binding low mana cost heals to same keys I use while tanking is working well with bigger heals bound separately.
I'm clearly not an expert in the matter, but I've always bound my heals to a 2nd bar when dual boxing. This way I can just press shift+1, shift+2, etc. to fire off a heal to the appropriate party member when needed. I can see why you'd like this automated in a 5 man setup and if you figure it out completely, I think that would be something anyone could benefit from.

Pally-Shaman-Tree would be awesomest should run half the regular and a few heroics.

DPS -> Using a DK or a Hunter is very easy as they're abilities are all (mostly) instant therefore fit into the same GCD rotation as the tank.
This was something I was debating as well. Akera, I really like your suggestion of taking a hunter as dps. They are easy to manage and the pet could provide for some nice off-tanking if needed. But is the loss of group buffs that a shaman/druid provide worth it? If the group consists of pally-hunter-? ... who should the healer be? Druid or Shaman?