From PHP.net today:

PHP 4.3.9RC1 has been released for testing. This is the first release candidate and should have a very low number of problems and/or bugs. Nevertheless, please download and test it as much as possible on real-life applications to uncover any remaining issues.

List of changes can be found in the NEWS file.

Since it's largely a bug fix release, upgrading isn't a must at this point, but it can always help. Besides, there are quite a few changes that were make for the better...