Hi. Thanks for the reply. I would post a screen shot, but I deleted that install (and cleared out the database) and when I reinstalled everything, that error/message wasn't generated. (Sorry.)
FWIW, when I got that message, it was on a WP installation that I had previously a) physically moved on my server from a subdir to the root (as per the recommendations for multisite use) and b) then upgraded both to beta2 (from b1) and then to multisite use. So who knows what might have gone wrong or unexpectedly in that likely too-clever process.
But I'm pretty sure the message was from WP, as the options to use sub-dirs or sub-doms were missing, and it simply informed me that I had to proceed with sub-doms -- that is, it seemed pretty official.
This probably acounted for the problem I asked about in this post...
...which also didn't recur once I did a proper fresh install.
Hmm. Oh well.
(As an aside, it's such a pleasure dealing with this kind of stuff with WP than it was with Movable Type. Now if only there was a reliable way to import from MT, to go along with WP3's cool new features....)