I just got this email notice from Obama's office and it states that Senator Obama will appear tonight at 10 p.m. Central/11 p.m. Eastern on Comedy Central's at the Daily show with Jon Stewart.
The Senator was scheduled to be in the studio with Jon, but had to be in Washington for Senate votes that came up this evening. He'll be on via satellite instead.
They rebroadcast it several times on Tuesday in case you miss it.