An interesting question came up in the WoW LiveJournal community early today: what does overhealing mean? At first I was a little surprised at the question, but then I realized they meant it not as "what is overhealing?" (to which the answer is that overhealing is the portion of your heals that would push the target past max health, and thus is in a sense "wasted"), but rather as "what does overhealing say about your healers?"
As a raid healer, my perspective agrees with most of the LiveJournal commenters: if your healer isn't having mana issues, don't worry too much about overhealing. It's going to be a natural consequence of proactive healing spells (Prayer of Mending, all HoTs, Earth Shield). In addition, most classes' big heals are too big to be used to their full extent all the time - to take an example from one of the comments, if I wait until targets are 10k in the hole before hitting them with something, we'll have a lot more deaths. Finally, sometimes overhealing can be actively helpful, as in the case of talents like Serendipity. I'd rather overheal by 1k than fill the hole exactly, because then I get a chunk of mana back.
However, if a healer is massively overhealing (the precise amounts to look for depend on the class), and is having mana issues, it can be a useful indicator that something is wrong with their playstyle - or they have a lot of latency. So, in summary: don't worry about it unless they're going OOM. Healing meters still suck, even the overhealing meters.