Brendon UPDATES: I'm not seeing any indication that Mr. Gaiman wrote the episode, Rich, but he has now confirmed via Twitter just what it was he was up to:
Went to the Marge Simpson Studios. Recorded my part as "Neil Gaiman", a British author. The accent was the hardest bit to get right.
Rich's original report: Looks like Neil Gaiman is going to be in The Simpsons. Indeed, looks like he may have written the episode in question as well, entitled The Book Job, thanks to these terribly cryptic clues Neil has tweeted…
Something like this, maybe?