This reminded me of the SNL skit. Clinton didn’t know that Palin was going to be there and learned about it from reporters:

Sen. Hillary Rodham Clinton has canceled an appearance at a New York rally next week after organizers blindsided her by inviting Republican vice presidential candidate and Alaska Gov. Sarah Palin, aides to the senator said Tuesday.
Several American Jewish groups plan a major rally outside the United Nations on Sept. 22 to protest against Iranian President Mahmoud Ahmadinejad.

I think it was a mistake for her to cancel, it would have been the media event of the day. Wall to wall coverage leading up to the event and then wall to wall analysis afterward.