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.
Unfortunately, I don't really see an easy solution here -- if you aren't in a position where you can tank, the best thing to do is to get in one. Either make buddies with a main tank who wants to switch out every once in a while, or start up your own guild and do the raidleading yourself. I haven't been a serious raid healer for a while, either, but I'd assume it's basically the same on that side -- at the highest levels, things probably get pretty cutthroat. And as long as there are limited spots for many more interested people, no amount of game balance will be able to change that. All you can do is perform as best you can and hope that when the opportunity comes along, you're there to take it.