It's beta. None of us know what things will look like. Maybe the bosses will hit for less, maybe the elementals will be able to take more abuse, maybe we will, maybe we won't. With the itemization changes, noone can predict what it will be like.

At the moment 5 shaman can do heroics, but in a gimmicky sort of way instead of the way the content was designed for. I'm not saying one way is more correct than the other, but the only thing we can be fairly sure of is that content will be doable with tank/healer/dps - as for the rest we can just hope for the best :-)

Either way, if you wan't to be on the safe side, level up a druid or paladin while waiting for wotlk.