In a post on the php.internals mailing list this morning Johannes Schlüter announced the availability of PHP 5.3.0 beta 1:

Please test it carefully, and report any bugs in the bug system, but only if you have a short reproducible test case.

The biggest change is dropping of OO functionality in closures as full OO support for closures is planed for a later release than PHP 5.3.0. See http://wiki.php.net/rfc/closures/removal-of-this

This release marks the begin of a feature freeze and bug fix only phase. If in doubt whether your change is a bugfix please run it by Lukas and me.

Downloads are available for:

Source:
http://downloads.php.net/johannes/php-5.3.0beta1.tar.bz2
http://downloads.php.net/johannes/php-5.3.0beta1.tar.gz
Windows Builds
http://windows.php.net/qa/