For those of you interested in attending a filming of Seinfeld, there was an article over the weekend in the LA Times about that show and several others. It is filmed at the CBS (not NBC!) Studio Complex, Studio City, CA. (Hence there is no reason why the inside of the diner should look like any existing one in NYC.) This is why he just bought a huge new hillside house in West Hollywood.
He does his own warm-up, doing stand-up and taking questions. The stand-up for the show is filmed separately, with probably several shows' worth done at one time. There was a number to call, but it was probably just the studio number, and I forgot to bring it.
John