I haven't played these characters since just before last Christmas. My two 80s have never seen the inside of ICC, and although my priest has gear from the ICC 5-man heroics, her gearscore is at that awkward 4700 phase - the one where no-one will take you to an ICC pug, but you really need Frost Emblems to upgrade your gear.
I probably won't actually play these chars much - it was more to drop in to say hi to my old guildies, but I thought while I was there I might as well do a random. So after I'd pummelled my UI back into shape, I queued up for a heroic. It was Halls of Stone (twitch).
My Nelf Druid (who used to be my "main" alt) doesn't even have Tier 9 pieces yet, so I thought I might chuck her in a heroic too. I didn't really do any healing with her from here before since I was terrified of the latency, but since last night it was only about 550ms I thought I'd give it a go.
Even though 550ms does mean a bit more red on the cast bar than I'm used to, I forgot that the server I'm dealing with is in Paris, which (sadly) is pretty much on the exact other side of the Earth. The average latency may be 550ms, but the actual data is subject to all sorts of laggy problems anywhere along the wires in between. As you can well imagine, that makes for some pretty hairy healing moments at times. Add to that low levels of haste and a tank who likes to pull all the mobs before Eadric in one go, and you have a stressed tree! Thankfully we got through it with only the loss of a Rogue.
So... healing with high latency. The main thing I can recommend is - be prepared. Most people have done all the content many times over by now - you may already have a fair idea of which bosses hit hard, or deal party-wide damage. If not, it will take practice.
For Druids, if mana is not a problem, keep the party hot-ed up. If you can, preempt party damage with a Wild Growth, but not too early because the first tick is the strongest. Keep a Regrowth ticking on the tank, and spam Nourish if you need to - hopefully that will cover lag spikes.
Other classes can follow the same sorts of guidelines - if you can cast HoTs, keep one on the tank. The rest is about predicting where the damage goes, and a whole lot of luck.
The hardest part about the whole thing is at lower levels of gear when mana is scarce. Then it becomes a balancing act between keeping everyone alive and trying not to run out of mana. Come Cataclysm, it's sounding like mana management is going to be much more important. Healing at high latency may just not be possible for much longer.