Sigh. I'm such a novice.
I cut and pasted from an old template into my WordPress 1.5.2 installation. Looked like it was working fine. Deleted old unnecessary backups.
Looks like my page doesn't validate. I didn't realize for some time because I was using ad-blocking software, set to "ignore errors". I disabled it the other day, and voila, html errors.
I've tracked it down to something in my sidebar; I suspect I either have a missing or an extra [li] tag someplace.
Would somebody with a fresh pair of eyes glance at it and see if my error is in an obvious spot? I haven't been able to find it. :(