Check out this new article Wizards of the Coast posted recently:

Ask Wizards: 11/24/2006

Q: Dear Sage,
If an ultimate magus (Complete Mage) with the ability to cast 9th-level spells as both a wizard and a sorcerer takes the Improved Spell Capacity feat (Epic Level Handbook), does he gain a 10th-level spell slot as a wizard, as a sorcerer, or in both classes?
--Chris