I think I knew about that and completely forgot about it.
I just don't pay as much attention to the spam problem as I once did a few years ago--it seems to be reasonably solved for me. I got over 600 spam messages to my primary address yesterday, and only 3 made it to my mailbox...and all I do is run SpamAssassin with almost entirely default settings and auto-white/black listing (which is Bayesian). Now, the primary things that motivate me are making the system more efficient with regard to resource usage, rather than more effective (though being more effective is also good). Some of our customers still seem to have problems, but I've not really figured out why SpamAssassin works so poorly for them and so well for me.
While it may be the case that PGs essay was many peoples first exposure to baysian spam filters, he didn't invent the technique.
The first time I heard of it was in a paper published in 1998 at Microsoft research. This predates 'A plan for spam' by 4 years. http://research.microsoft.com/~horvitz/junkfilter.htm