This morning I nuked my local CVS repository and started new from openacs.org:/cvsroot, and the install went fine and the site comes up. I'm mystified what happened, since there were no new commits to the tree between my last update last night and this morning. So whatever was going on was a problem in my local CVS respository (or something). Bizarre.
I've posted a bit more info about this ordeal at https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0006DJ&topic_id=12&topic=OpenACS%204%2e0%20Design.
Anyway, thanks all for your help, and sorry for wasting bandwidth on a self-solving problem.
