MySQL 5.1.54 packages are now available on Dotdeb in amd64/i386 flavours for Lenny and Squeeze.
As usual, please read the Changelog of this maintainance release before upgrading.
Now, about MySQL 5.5 (I was asked on Twitter or in the comments here), it will be available on Dotdeb as soon as it is in Debian Sid or experimental. Please be patient. If you don’t need some 5.5 specific features, such as semi-sync replication, and if you just need to boost your InnoDB performances, you really should take a look at your /etc/mysql/my.cnf file and enable the InnoDB plugin.