Do we KNOW for a fact whether shields are classified as an armour piece in Skyrim. I mean, it seems like a no-brainer question, but I'm wondering in terms of certain perks, like Mage Armour in the alteration tree, as to whether using a shield mitigates the mage armour perk - considering all the other armour related perks only seem to consider the chest/head/foot slots when determining whether or not the character is armoured.

Yes, shields are armor. Shields are not required for set bonuses because not everyone uses a shield. They may dual-wield or 2-hand. That shouldn't be penalized by the armor tree.

Of course, saying that now I see how that same argument could be used against shields hindering mage perks.

Exactly. :-/

Perhaps we can dual cast stoneflesh then switch to spellshield.

There is also the question if they affect mana regeneration (more so how much), since I keep hearing wearing armor lowers it, it may turn out that alteration just beats it because of that.

Another random thought came to mind... given there's now less distinction between weapons and armour, I wonder if you can poison your shield (Given you can use it as a weapon if you take the deadly bash perk)

And perhaps a cast on strike enchantment.