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Thanks for your response.

Yes, it's definitely a bug in Excel 2003 (as it also
complains about files it saved). I do not have a later
version of Excel to test.

Microsoft has a page about this issue that seems to say 2003
is the last version with that problem:
http://support.microsoft.com/kb/323626/
Their solution is worse than the problem, I'd be interested
in seeing how a later version behaves.

I know that if the first cell is quoted, Excel will open it
without complaining. I think the best solution would be to
quote the first cell if it starts with ID by introducing a
new QUOTE_ constant. I don't know how that part of the code
works (I'm too lazy to read things that aren't written in
python); maybe it's more reasonable to quote any field
starting with ID. I don't know of any other uses that would
break, but I'm not in touch with many csv users.