wp-db-backup produces a very good .sql file, but the .sql.gz file is not seen as an archive.
Configuration : Windows 64bit - Apache - PHP 5.3.4
Worked with PHP 5.2.x
Seems the gzencode() function does not work correctly.
The bug appears also with WordPress 3.0.5 and PHP 5.3 on a site located by OVH France. Worked with PHP 5.2.