Stole this from wowwiki and it pointed out something that I have observed a number of times. I "bolded" the pertinent parts imo.


What to do if healers/DPS draw aggro from the tank
If you do draw aggro, and you are not the tank, the first thing you should not do is run around. Stand where you are and use your threat reduction abilities: Fade, Feint, Feign Death, Invisibility, etc. Do not attack the mob if you want it to be pulled off of you quickly, and stop healing if at all possible. If the tank does not seen to notice it, type a quick 'help aggro' (or even 'agg') into the chat. Remember: every point of damage that you do to it and every point of healing you do makes you more threatening, and when something unwanted is attacking you, the last thing you want to do is further entrench yourself on its hate list.
If the tank is not nearby, help him to regain the aggro by running toward him. If he has to come to you, it makes it harder to maintain aggro on the other targets. Get close to him, let him taunt it off you, then move away and use any threat reduction abilities to help him keep it.
A hunter's pet (especially a beastmastery hunter's pet) is REALLY good for saving squishies who have drawn aggro. Turn on growl, and hit intimidation if you have it, and it should pull the mob right off (remember to turn growl back off when you send the pet back to the main tank's target).

Good info. The DPS classes, specially hunters with Mail Armor, should really be keeping an eye on the healer... the whole group should. If necessary non healers are better off dying than the healer.

Tank dies = Healer's fault
Healer dies = tank's fault
Dps dies = their own damn fault