There's no such thing as "power creep" in a roleplaying game. The term derives from and only possesses meaning in the context of collectible games - when what is considered "official" for tournament play changes with each new expansion, more powerful cards in later expansions creates "power creep" and forces players to keep up to remain competitive.
In D&D, however, the term has no meaning. New prestige classes can be overpowered and thus unbalanced, but there is no "power creep" because a) They are not mandatory and b) They can be altered, two things which are not true of the latest Magic: the Gathering or any other CCG expansion which may be overpowered.
So let us forget about that term, eh? Complain about balance all you like, but to call it "power creep" is silly. Especially since many of the most recent prestige classes are far from overpowered. Initiate of the Sevenfold Veil, anyone?