Arcane Schools: phenomenal idea. Cost/Benefit on specialists was way out of whack before... but
I now can prepare prohibited school spells at the expense of my Specialist benefits for the day. It's not that I don't think it's balanced— I just don't think anyone will ever take that bait. It is a rare adventure that would see me preparing my weakest spells in exchange for losing my best ones (think: Spell Focus, etc). Has anyone chosen to take this road while play-testing a specialist yet?
Clerics don't ever risk losing their domains, Sorcerers don't risk losing their bloodlines. I've happily avoided two (even 3!) schools of magic for a long time, I can't see myself making deciding to prepare them in advance at such expense, except in very rare and not-entertaining circumstances.
It would make more sense, I think, if prohibited schools took up higher level slots, maybe +1 or +2, like a metamagic feat. Still a hefty penalty, but I don't have to go against my completely character's theme just to prove that he's weaker in some schools than others.