My Druid is currently a Feral Tank, and I'm considering switching him to Restoration. Now, this is not at all because I hate tanking, but rather because I plan to make a Death Knight tank come WoTLK, and I want to try the healing game on my Druid for a while. I did a good amount of healing before BC, so trying that out again might be fun. Luckily, with my Druid, I've been able to collect a lot of good healing gear from Karazhan that no-one else wanted, and I'll probably buy a piece or two of healing gear with my spare badges, so I should be set to heal out of the gate when WoTLK hits.
Now my level 62 Paladin, I'm think of switching from Retribution to Protection. Again, this is not because I hate Retribution. It's a little more gear-dependent and random than I'd sometimes like, but it's a lot of fun, and I feel like Improved Sanctity Aura and Improved Seal of the Crusader actually let me bring a lot to a group. Still, I've always wanted to tank Shattered Halls with no CC, and It'll probably be a lot easier to get groups as a Paladin Tank.
The problem is, my Paladin has been much more of a side project for me, mostly leveled solo. I have very little spare gear. I have a few scattered pieces, but right now my best tanking weapon is Linken's Sword of Mastery, and my best shield is the Aegis of the Scarlet Commander, and I do not have a single piece of gear with any defense rating on it besides that shield, so I'm not optimistic about my chances of being able to get on right now, respec, and go effectively tank a Slave Pens run.
So right now it looks like my options are either to go buy a bunch of Green "Of the Champion" gear off the AH, try to tank with Retribution gear and hope my spec will pick up any slack, or level to 70, grab what tanking gear I can along the way, and hope the resilience on the level 70 PvP reputation set has enough resilience and stamina to let me fake being a tank until I can fill out my slots with PvE-focused tank gear. I'll probably plug away with the 3rd option for now and hope I can find some groups that understand what Retribution Paladins can bring to a party in the meantime.
I'm lucky, of course, because my Paladin is one of my many alts, and thus I have other options for grouping and playing, but I have to say, I'd feel sorry for the first time player or casual player who wants to switch a spec in midstream. If you need to switch roles, your old gear probably isn't going to cut it for your new spec, and if you don't really have immediate access to new gear, it can be a pain figuring out when - or if - to reroll.