Sitting at level 64 now and still going strong. Did mana tombs yesterday, which someone had experienced was the end of the viability of holy nova, but it went very well. Since some of the mobs do a mortal strike-type of debuff, I limited myself to pulling a max of two packs of elites (7-10 mobs typically), but it still went great. Two things though - the mana leeches are not to be AE'ed (obviously) and the AE fear mobs don't go well with lots of mortal strike debuff mobs. But I guess that's pretty obvious.

Anyway, I havent reached the point where mana starts being the limiter (I'm around 50% mana after I finish unless something goes wrong) and there's a new holy nova rank at level 68, although I don't expect to be able to continue doing this in northrend since the tendency seems to go towards mob packs with less, but harder hitting mobs. We'll see :-)