Most people in these forums have been reporting nearly a 20% benefit to DPS for elemental shamans in 3.0.8, which would erase most of the dps difference between them and elemental shamans. I suspect a boomkin is still a little bit better DPS at the moment but that it's no longer a huge issue.

If you are planning on running multiple shamans, Moonkin Aura is pretty useless. Because of the change in Elemental Oath in 3.0.8 (now gives 5% crit bonus when procced for group AND 10% damage bonus for the proccing shammy), you will want to have it on all your shamans, meaning that it will be up pretty much constantly. The 5% crit bonus from Moonkin Aura and Elemental Oath don't stack.

Druid also brings the following useful buffs:

1. Improved Fairie fire - 3% hit chance increase for party
2. Mark of the Wild and Thorns as minor static group buffs
3. Earth and Moon - group 13% spell damage done buff.
4. Insect swarm - boss debuff for 3% less hit chance

These are all pretty nice and a good reason to add a druid - Earth and Moon being the biggest bonus IMO. But you can also get the spell damage effect, in a multiple mob mode, with an unholy DK tank.

I have 5 shammies at 80 and a second group at 78/79 with a DK, 2 druids, pally and mage. I think I will probably run with DK, 3 shammies, and a druid for my main initial PvE/heroics group.