Build and skill sequence look fine. Masteries are good.

Since you have no mana regen in your build, you'll probably want Greater Seal of Scaling Mana Regeneration over Greater Seal of Replenishment. The mp5/level will catch up by the time you have mana problems if you use your passive effectively.

I would personally take Flash over Ghost on Nunu, his Blood Boil is already as good as ghost, plus attack speed. Flash is a far more useful summoner spell.

As for your item sequence, you should really show your order in your build, not just in text. It looks like you get boots after 2 full items.
Second, you should finish your Rod of Ages before building a second Catalyst, as RoA needs a lot of time to stack. Since Rod of Ages has the Catalyst passive, you may want to start with a Negatron Cloak on your Banshee's Veil.

There's my input. Also note that a guide with less than 5000 characters will not show up in the New Guide list or anything. I only found this through your profile.

Anyway I hope that answers your question. +Rep me if I was helpful at all, and feel free to check out my Alistar guide :)

EDIT: Don't fool yourself into thinking this is a tank build. It isn't, it's only a tough AP build. Just remember that when you play Nunu.