OK, we've solved it. Sorry to bother you all. The problem was that I
had some other boolean fields I was just ignoring. It used to work
fine when the back end was still in Access97. Apparently, when
Access97 sees a null in a boolean field it just assumes that it means
false, but when SQL Server sees such a thing, it causes this problem.
I was not filling in all the false values, and hadn't set the default
to false. Now I know.

Rebecca