The opening segment of Palin's appearance on Saturday Night Live last night, featuring both Tina Fey and Sarah Palin, is very funny. Palin's other segment, an appearance on Weekend Update for a Palin rap, is less funny (to me), but has its moments. I think both segments would've been better and funnier if they'd given Palin more to do and say, though.
I used to think that the resemblance between Fey and Palin was fairly superficial; I thought I could easily tell which one was which. But then I saw that opening segment, and I actually got confused about which one was which.
If you think you can easily tell them apart, take the Tina Fey or Sarah Palin Test. Nine screen captures showing one or the other; can you tell which is which? (To get your results, you have to specify your birthdate, but you can lie. Be sure to uncheck the checkbox where HelloQuizzy tries to get you to sign up to get mail from them. And in some browsers, you may have to scroll down a ways on the score page to find out your score, and it won't tell you which ones you got right. Still, a fun quiz.)
I think I can tell them apart better than I could earlier. If they're talking, it's relatively easy. In many/most stills, there are certain small giveaways. But it's harder than I would've thought.