I now suspect that it is simply a timing problem -- if you try to
update a cell too quickly in succession, it does not change at all.
Does that ring a bell? It's easy enough to test.

As for progress indicators, let me assure you that what I am doing
makes some sense. The user picks a subset of lines from about 40 on the
screen, and then hits "go" (presently a double-click on a cell, but
could be a dialog). The system then process a bunch of files
associated with each chosen line.

So, rather than a single, anonymous progress bar where you really have
no idea what is actually happening, I thought it would be nice to show
progress for each line's files *on that line* -- like "finding files
for line 3", "processing file 3.01", "processing file 3.02", etc, "Done
with 74 files for 3", then go on the "finding files for line 7" etc,
where the progress indicators would appear on the relevant lines of the
spreadsheet, right next to each selection.

Anyway, I got it working. Thanks for the good discussion on the topic.
I learned some things, even if they didn't solve the problem at hand.
If I check out the timing issue, I'll start a new thread.

Thanks. --David.