OK, I fixed it, but I don't understand how... :hehe:
In SSMS I right-clicked on the database, chose Properties, clicked on Files tab, and changed Owner to sa (it was formerly the SQL service account, which BTW is SYSADMIN on the instance).
I was then able to unmap the user login from the db_owner role on the database with no problems.
This solution doesn't make any sense, but it worked! Clearly there was some freaky dependency (bug?) tripping this up.
This link gave me the idea:
http://stackoverflow.com/questions/162764/sql-server-2005-how-to-change-dbo-login-name
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