Turns out that he chose it years ago as a unique identifier for himself, then later applied it to the webcomic.
This morning, that connected to a thought about my own quasi-unique online identifier: I often use "elysdir" (my middle name) as a username. Only when I Googled it the other day, I discovered that someone else (who claims to be, and may well be, an 84-year-old woman) uses it as an ID on MySpace. (And someone used it as an AIM ID long ago, though it's possible that was me and I just forgot the password.)
Anyway, so then I wondered: how long a string of non-accented modern English lowercase letters do you need if you want to assign a unique alphabetic ID of uniform length to every person in the US? That is, every person in the US gets a unique n-letter string; what's the minimum n that provides enough unique strings for everyone (and will continue to provide enough strings for at least another ten years or so)?
It would be easy to figure this out with a calculator, but I didn't have a calculator handy (I was in the shower). So, the puzzle (which is fairly easy) is to come up with an easy way to estimate the answer without needing a calculator and without having to do a lot of complicated multiplication in your head.
Ignore practical, social, privacy, and other constraints; this is just a math puzzle.
(That pop-up hint may not work in all browsers; sorry.)
P.S. added later: It turns out that I was misremembering the current number of people in the US, and the difference between what I thought it was and what it really is does change the actual answer, and makes it much harder for the estimate to match the answer. (Which is to say, it turns out we're currently just under the boundary between n and n+1 letters being needed, in such a way that my estimation method gives the answer n+1.) But in my original formulation of the question (in my head), I wanted to allow for future population growth, so I've now added a clause to the puzzle statement above to allow for ten years of projected US population growth, which resolves the boundary issue.