During the last weeks, MySQL suffered from many vulnerabilities (see DSA-2429-1 and DSA-2496-1). Due to the non-disclosure of security patch information from Oracle, the Debian security team is now forced to ship upstreams version update of MySQL 5.1 (5.1.63 as of this writing).
Then it makes no sense for Dotdeb to duplicate this effort and to maintain the same versions of MySQL 5.1 for Squeeze with the same features and level of optimization. As a consequence the MySQL 5.1 packages has been removed from the Dotdeb repository (you’ll find archived versions here).
Please just be sure to have debian-security in your sources.list to have the most recent security fixes for your MySQL 5.1 server :
deb http://security.debian.org/ squeeze/updates main contrib non-free
Now, don’t worry, if you want to install a recent MySQL 5.5 server, the packages will still be actively maintained on Dotdeb, in terms of features and optimization.