...I doubt it. I really really doubt it. But over on the IMDB's trivia page for The Big Lebowski, there's this:

Screenwriter Charlie Kaufman makes a background cameo appearance during the interpretive dance scene as another member of the theater audience. Kaufman's own script, Being John Malkovich (1999), also contains an interpretive dance sequence.

I say baloney - I doubt Charlie's ever done, or will do, a cameo, ever - but I've been wrong before. I'm sure I have. Also, Lebowski came out the year before Malkovich, Kaufman's first film. So a cameo at that early stage, in particular, seems unlikely to me.

Eric has watched the DVD and says it looks like Charlie, though it's hard to tell. Here's a screen cap of the offender (I don't mean Eric). Call me stupid, but it looks more like John Goodman to me.

(I'm kidding about Goodman. Obviously it's Jeff Bridges.)

Then again, maybe it's Jon Glaser.