i would say that if a prestige class does not say that casting divine spells is a prerequisite, and it gives domains/divine spells, i would start the character at casting level 1 cleric. what does that hurt? at the verry least, i would give the character the granted power of the domains (after all, he is taking a prestige class where the main benefit is probably spells) with that said, all of the prestige classes i have seen say +1 level of existing spellcasting class or something similar. HOWEVER if a player in one of my games took a single level in the class just because he wanted the granted power of the prestige domains, i would require some really nice roleplaying to not lose them.