Moodle 1.5.4 is now available from the Moodle download page.
This release just wraps up many small bug fixes that have been made in Moodle since the release of 1.5.3 six months ago. If you are using a recent 1.5.3+ then it's likely that there is little difference to what you have. However there are a number of security fixes wrapped in this release so upgrading is always recommended. You can read the full release notes for 1.5.4 for details of the bugs fixed.
Note that the release of Moodle 1.6 (with many new features) is imminent.