I'd like it if you couldn't queue for a dungeon as a tank or healer whilst in your DPS spec, but I'm not sure how feasible it is to implement this and there are potential hiccups (see below).

If MoP is going to remove a lot of tanking and healing abilities from DPS hybrids, something like this needs to happen. I've already seen three very confused new players try to tank SFK as a shield-less DPS warrior and it can hardly be much fun for them to have the group shouting at them. It certainly wasn't fun to heal.

I can see potential problems with this idea. What if you're a druid, levelling as balance/resto? If you're out questing while you wait for a dungeon pop, you'd have the choice of killing things more slowly in you healing spec, or sitting in a long queue as balance.

A possible workaround - which might be far too complex to implement, I don't know - would be to allow players to queue for roles appropriate to both their specs. Before level 30, your ret paladin can only queue as DPS, but after 30 when he's dual-specced for prot, he can queue as a tank as well.