The SF Sketchfest website has been updated with the following:

Due to a previously unforseen engagement out of the country that cannot be rescheduled, Neil Patrick Harris has regretfully canceled his appearance at SF Sketchfest, and has been replaced by Jason Segel.

Neil followed this up with a statement of his own last night:

“I’m so terribly disappointed to be missing SK Sketchfest this year, and I did everything in my power to avoid having to withdraw. I’m a huge fan of the festival, and I love the Bay Area, so needless to say, I can’t wait to come back… assuming San Francisco will still have me! Meanwhile, I managed to wrangle Super A-List Movie Star and Sometimes Nudist Jason Segel to join the show in my stead. He’s great. And single. So if you play your card right, you may just get invited to the “after party”, if you know what I mean…”

Apparently refunds are available, so if you purchased a ticket and no longer wish to attend, get in touch with Sketchfest or the Cobb Comedy Club directly and they should be able to help you out.