Shockadin in NorthrendFollow

Is the Holy/Shockadin spec viable for questing in Northrend, or do we all have to go Ret if we want to level quickly?

You're really asking two different questions here. Yes, Holy will be viable, but yes, you will probably have to go Ret if you want to level quickest.

Part of me wants to stay Holy til 80, since my PVE Holy set now is a bit better than my PVP Holy set and can conceivably even out-DPS my fledgling Ret set. A lot of it may depend on what my friends opt to do though: if 3+ of us can take the boat to Northrend together on the 13th, I'll be a happy little pocket-healer for as long as it lasts. Otherwise, I know I can switch to Ret for faster individual leveling, while having an off-spec healing set that's more than capable of coping with pre-80 content. The big dilemma for me this week is whether I should spend the last of my BoJ to pick up a Blade of the Harbingers for the latter contingency.

Not really, Prot outlevelled Ret from lvl30 all the way to 70 for me, I beat every Retadin in my guild. Multi-pulls and AoE farming with no downtime was a case of slow 'n' steady wins the race; nuking like hell, healing and then having to drink every three mobs was Ret's major downfall.

Prot's levelling potential would seem quite high IF you can effectively AoE grind; I just don't think you'll be able to as everyone will be fighting over mobs for about three months.

Holy... no idea to be honest. I imagine a Shockadin spec would cause solid damage and Divine Plea should minimise mana downtime, but you'd need to keep your crit% very high to get good enough mana efficiency.