I think they could be a very good contender in large BGs. In cata BGs will be important and you won't have to arena to get top gear.

Out of the aoe magic classes, moonkins seem the most viable. You have stealth, armor, knockback, and heals. Mages are way too reliant on mobility (multibox blink and kiting? no way). Warlocks have some viability, good nukes like shaman (but not quite as reliable or hard hitting), dot and run (easily healed through, I did it back at 70 it was frustrating), and some rain of fire, SoC gimmickery in AV, but other than that I'm worried they'll just fall over like mages. Shadow priests seem fun, but they go oom, only have a few spells, and as far as being able to survive a barrage of melee and crap, they are worse than shaman.

Arenas... still looks like shaman are king, and dk+ret melee can get you 1800 or maybe even 2000 if you want to bang your head against the wall for months. No one has done much with moonkins in arena yet, I don't think they have the on demand burst they need and the tools that shaman have. but you could try