Yesterday, many announcements have been made on MySQL Sunday. MySQL 5.5 is now RC, and will be GA soon, with many (exciting) improvements.
No, there are no MySQL 5.5 packages on Dotdeb (yet), but MySQL 5.1.50 packages for Debian 5.0 « Lenny » are now available on Dotdeb in amd64/i386 flavours.
This bugfix release embeds the InnoDB plugin 1.0.11.
As usual, please read the Changelog carefully before upgrading.