A bug fix release is available on http://marathon.sourceforge.net/
The primary reason for this patch release is to add a stable release with Mac OS X Lion compatibility. Many other bug fixes are also included, so all users of Aleph One 0.23.2 are encouraged to upgrade.
Users of the latest beta release already have the bug fixes present in 0.23.3 (and more), so they should not upgrade. Lion users running the latest beta release should be OK as long as they keep OpenGL enabled.
You can read the release notes for 0.23.3 directly on SourceForge: