I just thought of a slight problem. Seasong, you stated that the main issue with higher levels would be making it so that staying in the class would be a viable option, but with some of the cleric-based PrC's I think that line is lost.
The Divine Oracle for example from Defenders of the Faith, could be attained by this sorceror and would gain all levels of spellcasting, plus:
Divination Prestige Domain
+2 to Scry checks
Presceint Sense (like Evasion but any armor)
Divination Enhancement (increases % chance of being correct)
Uncanny Dodge
Never Surprised
keeps a d6 hit die, less skill points, and only loses two maximized spells a day. It seems that this would make the prestige class the only choice for a divination oriented sorceror (true strike, anyone?)
The Sacred Exorcist from DotF would be an incredibly powerful character combined with ten levels of this alt.sorceror.
d8 HD
10 levels of spells
Only 2 skill points
Exorcism Prestige Domain
Chosen Foe(similar to ranger)
Turn undead as a cleric (level-2)
Extra Turning
Detect Evil
Dispel Evil
Consecrated Presence
If the class were divine, it would be open to even further abuse, due to the fact that most divine classes are specfically worded to only allow additional spell levels in a divine spellcasting class (two two above do not, however).
I still like the class alot, and think that we need to keep working on something to make the high levels more interesting. Maybe some sort of extended Eschew Materials that would allow higher cost materials to be converted into XP costs or some other sort of payment.
~hf