Druids are viable as MT for both 5 mans and raids. Gear upgrades are rarer at high level raiding for druids though, so there is concern that we won't scale as well as warriors (who do get lots of high level gear upgrades).
Personally I'd rather be OT than MT and once my mob is dead I can shift to catform and help dps (maybe innervate or CR on the way there if needed). MT is doable, but you lack the ability to role shift which is one of the things I love about my druid.
Druids can MT, prot warriors can't really dps though, so best to have the druid OT and then dps when needed in my opinion.
Edit: Offtanks
Basically some encounters have more than one mob in the boss fight, or a boss that requires two tanks.
Examples from Karazhan:
Attuman - You start the fight by attacking attumans horse Midnight, after a couple of seconds Attuman spawns (so now we have 2 mobs to kill). When one of them gets to 40% Attuman mounts up, leaving you with one (mounted) mob to kill. So here the offtank pulls and tanks midnight, the main tank grabs attuman when he spawns and then again when he mounts up. The offtank then helps with dps.
Moroes - This boss gouges the tank, so you can either have the tank get agro and then turn to face the wall (not a good idea though cause they can't keep building threat) or you can have an offtank. Fo rthis sort of fight the offtank maintains a high agro on the boss, but not enough to pull it off the main tank. Then when the MT gets gouged Moroes will hit the offtank instead of running off to whack the healers.
Another popular one is bosses that have stacking debuffs (that reduce armour or healing taken), at max stacks you end up with a tank who receives 0% of any healing done to them so here you'd want one or more offtanks so they can each take a few stacks of the debuff, but let someone else take over before it stacks too much (and they can wait for it to fade off of them).
Edit Again:
Warriors tend to make better tanks vs casters as they can spell reflect and get a reduction in spell damage taken from defensive stance. Druids generally have higher hp than warriors so can 'soak up' the damage better, but this may pout more of a strain on the healers mana.
Edited, Jun 4th 2007 7:22am by Tynuv
Edited, Jun 4th 2007 7:28am by Tynuv