In my opinion, incidences of the number 23 in modern works of art don't count as part of the supposed Enigma, for the simple reason that just about everyone with any kind of (geeky/intellectual/artistic/conspiracy theorist) leanings has already heard of the 23 Enigma and may be intentionally using the number in their own work. It may be an in-joke, it may be done just to make the author look clever, or it might even be a subconscious decision.

"In the fantastic film "Rushmore", Max Fischer's phone number is "Extension 23" - AT BOTH SCHOOLS!! Now how creepy is that?" Er, not very, not too creepy at all, because "Rushmore" was written by a couple of guys who almost certainly read either Burroughs's or RAW's ideas on the subject. Furthermore, it was an imaginary extension invented by the ultra-geeky hero Max, and Wilson and Anderson probably thought it would be just the kind of mysterious concept Max would revel in.

On a similar note, now that I think about it, I'm pretty sure I've used the number in one or two of my own stories.

Not to mention the fact that Burroughs was pretty messed up, and Shea and RAW simply revel in being or sounding messed up. Of course, that doesn't invalidate the theory, but I thought I should point it out, if only for the record.