If this has been debated before (ad nausuem), I apologize for more nauseum. 
OK - three related questions, each more perilous than before:
Jane is a Rogue 5, Wizard 3, Assassin 5.
She takes a level of Arcane Trickster, which gains her +1 spellcasting level in another spellcasting class. She qualifies for this by being able to cast 3rd level Assassin spells, which are Arcane.
Can she choose Assassin as that spellcasting class? It has it's own progression and spell list, and so is a 'spellcasting class', albeit a Prestige Class.
If that is legal, what happens when she's a Rogue 5, Wizard 3, Assassin 10, Arcane Trickster 1, and she is technically a Level 11 Assassin with regards to spellcasting? Does she gain no further progression, and just has a greater effective spellcaster level on her Assassin spells?
Ok, next. Bob is a Wizard 6, Alienist 6.
He takes a level of Loremaster, which grants a spellcasting level of a previous spellcasting class. If he chooses Alienist as that previous spellcasting class (which also grants a spellcasting level of a previous spellcasting class), that functionally just shunts back through to Wizard and grants him a spellcasting level of Wizard.
Now, our piece de resistance:
Gertrude is a Cleric 5, Wizard 5, Mystic Theurge 5.
She takes a level of Loremaster, which grants a spellcasting level of a previous spellcasting class. If she chooses Mystic Theurge as that class - well, that's a level of *both* a Arcane and Divine class. Functionally, it could shunt back and split into both a casting level of wizard and cleric.
Now, my ruling and explanation of all this is that you can only name a class or prestige class that has it's own spellcasting progression as a class that you choose to raise your spellcasting level in, so Jane would be OK taking a caster level of Assassin, but Gertrude has to go and choose either Wizard or Cleric, and Bob has to choose Wizard.
I may be wrong on being able to choose a prestige class with progression as a valid choice as well, so I'd like to hear if I am.
I'd also like to know if I'm wrong on the Mystic Theurge deal. While I'm pretty secure in my semantics, it would give solace to the people that decry the Theurge as weak that they could be a Clr3/Wiz3/Theurge10/Loremaster4 and cast as a 17th Wiz/Clr, I suppose, if it did stack like that.
On a slightly related question, I'd assume one could take the Practiced Spellcaster feat and apply it to a prestige class - so Jane could up the caster level on her Assassin spells. It seems like an awfully smart use of the feat for a dedicated Assassin with no other spellcasting. (and, also, one would not be able to apply Practiced Spellcaster to Mystic Theurge, or any other Prestige Class that does not have it's own progression, I'd assume)
Anyway. Thanks for all opinions.
Koewn

OK - three related questions, each more perilous than before:
Jane is a Rogue 5, Wizard 3, Assassin 5.
She takes a level of Arcane Trickster, which gains her +1 spellcasting level in another spellcasting class. She qualifies for this by being able to cast 3rd level Assassin spells, which are Arcane.
Can she choose Assassin as that spellcasting class? It has it's own progression and spell list, and so is a 'spellcasting class', albeit a Prestige Class.
If that is legal, what happens when she's a Rogue 5, Wizard 3, Assassin 10, Arcane Trickster 1, and she is technically a Level 11 Assassin with regards to spellcasting? Does she gain no further progression, and just has a greater effective spellcaster level on her Assassin spells?
Ok, next. Bob is a Wizard 6, Alienist 6.
He takes a level of Loremaster, which grants a spellcasting level of a previous spellcasting class. If he chooses Alienist as that previous spellcasting class (which also grants a spellcasting level of a previous spellcasting class), that functionally just shunts back through to Wizard and grants him a spellcasting level of Wizard.
Now, our piece de resistance:
Gertrude is a Cleric 5, Wizard 5, Mystic Theurge 5.
She takes a level of Loremaster, which grants a spellcasting level of a previous spellcasting class. If she chooses Mystic Theurge as that class - well, that's a level of *both* a Arcane and Divine class. Functionally, it could shunt back and split into both a casting level of wizard and cleric.
Now, my ruling and explanation of all this is that you can only name a class or prestige class that has it's own spellcasting progression as a class that you choose to raise your spellcasting level in, so Jane would be OK taking a caster level of Assassin, but Gertrude has to go and choose either Wizard or Cleric, and Bob has to choose Wizard.
I may be wrong on being able to choose a prestige class with progression as a valid choice as well, so I'd like to hear if I am.
I'd also like to know if I'm wrong on the Mystic Theurge deal. While I'm pretty secure in my semantics, it would give solace to the people that decry the Theurge as weak that they could be a Clr3/Wiz3/Theurge10/Loremaster4 and cast as a 17th Wiz/Clr, I suppose, if it did stack like that.
On a slightly related question, I'd assume one could take the Practiced Spellcaster feat and apply it to a prestige class - so Jane could up the caster level on her Assassin spells. It seems like an awfully smart use of the feat for a dedicated Assassin with no other spellcasting. (and, also, one would not be able to apply Practiced Spellcaster to Mystic Theurge, or any other Prestige Class that does not have it's own progression, I'd assume)
Anyway. Thanks for all opinions.
Koewn