Rails uses the MySQL x’FFFFFFFF’ syntax for BLOBs, which was introduced in MySQL 4.0. Therefore Rails doesn’t support working with BLOB fields in MySQL 3. This wouldn’t be a huge problem, but there’s a large installed base of MySQL 3 users: Fedora Core 3 and RedHat 9 both use MySQL 3.
Fortunately, the upgrade from MySQL 3 to 4 is fairly painless; you can get MySQL RPMs that work for these systems at the MySQL web site.