I'm with you on this one, though I can see the argument for the other side too. Look at the phrasing:
Casting Defensively: If you want to cast a spell without
provoking any attacks of opportunity, you must make a
concentration check (DC 15 + double the level of the spell
you’re casting)to succeed. You lose the spell if you fail.
It is essentially an if-then statement. You wish to cast without provoking attacks of opportunity. In order to do this, make a check. This suggests that failing the check means you fail in your attempt to cast defensively.
However, the addition of the last sentence suggests that losing the spell is the consequence when you fail the check. Furthermore, I'd argue that when casting defensively you are essentially on the look-out for any danger, which means that when you fail the check you basically have to stop concentrating fully on your spell to avoid the AoO and thus lose it.
My opinion would be no AoO.
If the caster casts a spell he provokes AoO. If he loses the spell.. then he's no longer 'Casting' ;) He's just standing there waving his arms around :P
But in a more serious note... AoO can be REALLY mean in general. It's bad enough for the character to have just wasted a round (and a spell) for absolutely nothing... but to then hit him with an AoO is just salt in the wound.
If the AoO guy actually got the caster to lose his spell... then his job is done!
Spellbreaker (Combat)
You can strike at enemy spellcasters who fail to cast defensively when you threaten them.
Prerequisites: Disruptive, 10th-level fighter.
Benefit: Enemies in your threatened area that fail their checks to cast spells defensively provoke attacks of opportunity from you.
Normal: Enemies that fail to cast spells defensively do not provoke attacks of opportunity.
I think this settles it.
What the others said.
It's in the core.