I figured this definitely can't be done with two-handed weapons, but what if a caster is using one hand to cast for the somatic component to cast a spell that takes 1 round, and the other hand with a melee weapon letting him take attacks of opportunities? I figured this would be using if some other caster decides to cast a touch spell right in front of him/her (not casting defensively for whatever reason) or if someone decided to ranged attack right next to him/her. Does this work?
If a spellcaster did this, I would assume that he was voluntarily giving up his concentration to do so and the spell would fail.
No assumptions necessary.
This is what you're looking for:
You only provoke attacks of opportunity when you begin casting a spell, even though you might continue casting for at least 1 full round. While casting a spell, you don't threaten any squares around you.