[Gimli]

I don't think any items will outright prevent you from casting spells. But if you're casting Arcane spells, then armor and shields will sometimes cause spell failures (right click the armor/shield in question to see how high a % chance of arcane spell failure they bring with them). The armored arcana feats can reduce that % chance, but IMHO it's not really worth it, just don't use armor or shields with an arcane caster (or at most, only use a small shield and take one rank of AA).