I haven't read the article but I think I can see where he's coming from. The developer market is pretty much saturated as far as marketing is concerned. If you're a developer you know who Joel is. So continuing to blog won't yield as many customers as spending time on marketing to other segments.
I personally think it would be smarter to slow his posting frequency rather than quit all together but anyone whose read Joel's blog knows he has a certain love for the dramatic.
I went to one of his StackOverflow Dev Days. While I didn't speak to him, I caught glimpses of him every so often standing by himself amongst a sea of developers that made me think that he's getting a bit bored with... things... for lack of a better term. I'm not sure whether "things" would be his business, software development, or a combination of things in his life. The problem with being Joel from JoelOnSoftware is that you have to always be on. You can't ever have an opinion about anything without getting into a discussion with someone about it. Like you, I hope he just goes on sabbatical.
Blogging/writing on a regular basis is a lot like polling vs event-driven programming. Joel's been polling himself way too often trying to come up with something to write so that he can publish something. When you poll for something, 90% of the time you'll find the thing you're precisely NOT interested in. I'm sure he'll be back when there's something good to talk about.
I just looked and it actually seems like he picked up posting a bit more than he used to. For most of 2009 and maybe earlier, it was just product announcements and links to his column (which always seemed dumbed down.)
I personally think it would be smarter to slow his posting frequency rather than quit all together but anyone whose read Joel's blog knows he has a certain love for the dramatic.