http://eu.battle.net/wow/en/game/patch-notes/2-0-1
HoTs from different sources will now stack (i.e. multiple druids can cast the Rejuvenation spell on 1 target).
I would like to note that I have seen indicated in a few different sources that there is a limit of 3 stacks of the same buff, regardless of the source.
http://www.wowhead.com/spell=139/renew#comments
"Sware on 2006/12/13 (Patch 2.0.1)
I read somewhere only 3 of the same buff can stack.

Sprite on 2007/05/27 (Patch 2.1.0)
Correct only 3 of the same buff can stack so only 3 renews can be on one person at the same time. if a forth is cast it wont have any effect and will jsut waste mana."
I haven't been able to find any particularly credible sources on this but I wanted to mention it for your consideration.