I defy you to tell the difference between her character and the Governor of Alaska, who has been busy lowering expectations all week. The main difference, of course, is that Fey is still in front of TV cameras, while Palin can no longer be found. Anywhere.
And the most brilliant stroke: Palin herself provided much of the material. She writes her own comedy and all Fey has to do is perform it the way Palin does. Fey isn't doing a caricature (like Dana Carvey's George HW Bush), but is giving a performance of uncanny accuracy (closer to, say, Helen Mirren in "The Queen").