It's been noted (thanks faejon) that IE 5.5 has an issue with HTTP content-disposition header. This results in IE 5.5 incorrectly downloading the DB backup as a .html rather than .sql or .gz.
The relevant knowledge base article is:
http://support.microsoft.com/support/kb ... 9/6/67.asp
and the solution is fully described in:
http://support.microsoft.com/support/kb ... 1/1/19.ASP
This is not a bug in phpBB 2.0.