Moodle 1.5.2 is now ready for use! As the release notes show, a number of bugs from 1.5.1 have been fixed, particularly in the Assignment module.
If you are running Moodle production servers of any kind then I highly recommend upgrading to this version.
For information about creating and upgrading Themes for Moodle 1.5.x, see this new documentation about Moodle Themes.