Whoops. I missed the "to cast cleric spells" part you'd had and thought you were just talking about Mystic Theurge requirements in general. My bad.
As others said it's possible with Favored Soul (basically a divine sorcerer), and Druid has most of the same spells as Cleric (also true for Rangers, btw, but a significantly smaller list). But yeah, I just messed up my reading comprehension.