I found mention of this apparently infamous O'Reilly banner ad when flipping through my recently purchased copy of Programming Python. The ad is hilarious, and apparently pissed off enough Perl zombies to garner an apology from O'Reilly.
Here's the apologetic posting from O'Reilly (found in the above thread):
Hello all, I'm Allen Noren, managing editor of the O'Reilly Web sites. Tim forwarded me email from each of you about the Python ad on Slashdot. I want you to know that I am responsible for the ad. and that it is not part of a corporate strategy to fan the Perl/Python religious wars. On the contrary, it was simply an attempt to instill some levity onto those flames as we introduced an excellent new book. Set in the context of Slashdot I thought people would get the joke. Of course O'Reilly is not dissing Perl or promoting Python over Perl. We don't diss anyone or their operating systems or their preferred language. I'm sorry if it was taken that way. Thanks for taking the time to read this and for voicing your concerns. --Allen