I also really like the way AU does things...
There are some big potential problems with making all spellcasting spontaneous.
One, you make prep casters much more powerful. Not just wizards, but clerics, too!
Two, sorcerers are significantly weaker. This might not bug you too much, but you also make bards significantly weaker. Both classes rely on their freedom to cast as part of their 'power.'
I've seen these effects in a spell point system (which magnified the problem even more), and it wasn't pretty. Clerics... god.
One thing I'm curious about... do your prep casters regularly leave slots open? If they find themselves strapped for spells, maybe they need some reminders. By leaving a few spells unprepped, they can get whatever they need, as it comes up. Sure, there's a delay, but in many cases pulling out the books (or the prayer beads) to prepare water breathing isn't a rush issue. 15 minutes is fine.
In short, prep casters can have the freedom to cast any of their spells, provided they leave slots open and take a little time to fill up when needed. In fact, I'd suggest that all prep casters leave at least 1/3 of their daily allotment unfilled, unless they know absolutely that they are going to hit a big confrontation. And even then, at higher levels, fill up your highest spell levels, but leave the lower level spell slots more unfilled. Lower level spells are less likely to be key in a conflict, and more likely to be handy in some random and circumstantial scene.
If you are worried about wizard spellbooks, I'd recommend instituting some sort of 'study book' rule. I personally hate the issue of 'ok, wizard has this vital piece of stuff. If the DM screws with it, it can make the situation suck mightily.' I don't like that kind of arbitrary screw. So instead, the wizard's 'valuable' spellbook sits at home or in a safe vault at his guild, while he refreshes his memory with a cheap set of notes. If you want to limit this somewhat, perhaps limit his notes to the number of slots he has. (This is somewhat like the Ready thing from AU)
If you do choose to make things non-prep, my suggestion is to model all casters on sorcerers, though possibly switch slots with learned for some. That is, switch 6 slots and 4 learned with 4 slots and 6 learned.
So a cleric would be a sorcerer with Wis as a key ability and different skills, and a cleric spell list, turn/rebuke instead of familiar, and domains. But low BAB, saves, etc.
Wizard, sorcerer based on Int.
Druid would be altered tremendously. Balance-wise, I'd use sorcerer with Wis and druid list, skills, and limitations and swap slots with learned. Animal companion instead of familiar. I personally think druids are a little too powerful in 3.5, but opinions obviously differ.
Anyhow, some ideas.