Exciting news: PHP 5.4 is out. Thanks to the release managers Stas & David for making it happen. The PHP code is syncing to mirrors. PHP documentation will take a day or two to build & sync. Take a look when it reaches you.
The announcement lists some of the features that have changed.
There is a current tidy up of documentation for new features and migration happening. There are some low priority bug fixes that are waiting merge. These came in during the extended release process and were left out to avoid destabilizing the code.
Some PECL extensions still need to make PHP 5.4 compatible releases. Rasmus helped out by making changes to some extension, but the package owners need to bundle up releases. Other extensions may need some work.
XDebug is due for a compatible release soon.
PHPUnit is said to be fine already.
APC has at least one pending fix too, for the "?:" operator.
In summary, PHP 5.4 is available and I expect this branch will stabilize quickly.