Multiple Prestige Classes(Caster 3.5)

[Bayushi]

Lets say I have lvs in Mystic Theurge and then take a level of red wizard.
Red wizard gives +1 level to existing class spellcasting.
Mystic Theurge is a arcane spellcasting.
Is there any thing stopping a a player from advancing mystic theurge casting ability(which gives +1 to divine and arcane)
there isnt an infinite loop because you must take a lv in a class you Had before you started the prestige class.

Anyone have an official(book or wizard) ruling on this?

[Troacctid]

Advancing the spellcasting of Mystic Theurge does nothing, as Mystic Theurge does not offer spellcasting, it merely improves the spellcasting of other classes.

[Psyren]

Anyone have an official(book or wizard) ruling on this?

Yes - the book says "spellcasting class." Mystic Theurge is not a spellcasting class, because it has no spellcasting of its own. Therefore it has no casting for another PrC to advance and there is no loop.