I've been tangling with the tough issue of tanking again lately -- I have finally leveled up my paladin to 80, and ideally, the plan is to jump in as a dual specced tank and healer when patch 3.3 hits. All LFG all the time, eventually headed to higher level raiding. But Honor's Code has a good post up this weekend about the trouble that many tanks are facing lately -- they say that while there's lots of tanking to be had in the early endgame (every 5-man and heroic group out there needs a tank), the available positions narrow down as you get farther up. By the time you're reaching Icecrown (which I would like to do someday), there are so relatively few guilds raiding there and so few serious tanking spots within those guilds that you either have to be a really great tank, know someone who's in charge, or be ready to switch off to another spec or alt when necessary.
In essence, they're saying there's a glass ceiling for tanks. Once you reach a certain point, it's hard to find even the opportunity to be a solid tank.