The database file needs to be a seekable file, so /dev/null will not work. I guess I can add a check in Maemo Mapper to make sure the file is seekable. I don't know how the GDBM initialization would have worked on /dev/null, though, and that would have produced an error before causing Maemo Mapper to crash. In any case, it's a bug that I should fix.

By the way, if you're trying to go without a map cache entirely, just leave the field empty. Maemo Mapper works perfectly fine in that configuration. It just has to download maps every time it redraws the screen.