From what i know, there are 2 options to level a priest/healer class:
1. you do it on your own and go into dmg talents (shadow for priests, feral druids etc..)
2. you do a lot of instances as holy
In TBC we will need to do the 5-man instances a lot to complete mandatory quests/rep/items to progress in outlands, therefore if any priest could be persuaded into going holy again, i wouldn't mind at all.
However if healers want to go dmg, i'm afraid you picked the wrong class... tho they are a great asset in damage output, there are better classes out there that are made for that job (rogues, mages,locks..)
Except for the druid/pally/shammy, which was meant to be a versatile class that should be able to adapt to the situation (healing/dmg/tanking)
just my 2 cents.
Bottom line offcourse is that it's mandatory to spec the way you really like to play. If you're not want to heal and *have* to do it, you won't heal as good as someone who loves this aspect of the game
I saw the power of resto druids already and i can tell you.. they don't have to feel less vs any other healing type, if not superior to some of them
However without good healing, you won't get very far in the lvl 70 rush, unless you don't mind mindless grinding of the same mobs for some months...