Does the monk AC disappear during attacks of opportunity(like the dex bonus), like when you're casting a spell? And does uncanny dodge negate this? I have no idea what 'flat-footed' means.

Flat-footed is just a term used, when your character looses his/her Dexterity bonus to AC and cannot make attacks of opportunity.

The dexterity bonus does not dissappear when an attack of opportunity is made. In some situations your character looses his/her Dexterity bonus to AC, but the monk keeps his/her Wisdom bonus to AC.

Uncanny Dodge makes you keep your Dex bonus to AC, but this is not something a monk gets, and it isn't selectable as a feat. If you want Uncanny Dodge, you either have to take Barbarian or Rogue levels, but that means you cannot gain any more level with the monk.

The monk's AC bonus due to level is not lost due to being flat-footed either. (+1 AC per monk level/5, rounded down)

Unless they've changed it with SoU/1.30, in NWN you can mix monk and/or paladin levels just like any other class. This is different than PnP.

You still cannot easily mix monk and barbarian classes since one must be lawful and the other cannot be lawful. ;)

What about when you're spellcasting? Do you lose it during that time?

Monks can cast spells? :eek:
Learn a new thing each day.

Or are we talking about Multi class monks?

Monks do not cast spells. Casting a spell provokes an attack of opportunity. Should a monk cast a spell, he keeps his normal AC (With Dex modifier and Wis modifier), but the opponent may make an attack of opportunity. If the opponent hits, the spell caster must make a concentration check to keep the spell.

Just remember this: When an attack of opportunity is made, you always keep your normal AC.

Actually, IIRC, casting a quickened spell does not trigger an attack of opportunity. Otherwise, I believe you are 100% correct. :)