It might be worth pointing out that Dancho is far more intelligent and more educated than average, and so perhaps better able to moderate schizophrenia. Its symptoms may appear more subtle to observers, being filtered through a more rational intellect, which might make it more difficult to spot.
All of this is speculation of course and not terribly helpful without more information on Dancho's whereabouts and activities. I hope he's ok.
I scanned through his blog (which is a good read) and couldn't find anything that somebody would kill him or make him disappear over.
I would be more interested in knowing why it took ZDNet 6 months to report one of their bloggers missing?
Also there is no way to corroborate if this letter is in good faith. This could be anyone sending this letter. It could be complete fabrication designed to send a "hes gone crazy" message.
I find this whole situation suspicious.
It's quite possibly a script kiddie successfully trolling the world, or a serious professional suffering some sort of psychotic break, or there may actually be something sinister going on. Better safe than sorry, I guess - I'm not entirely sure what to do with this information though besides continuing to raise awareness.
The more relevant question is why bug the bathroom?