For users upgrading from PHP 5.3 there is a migration guide available here, detailing the changes between those releases and PHP 5.4.0.
Further details about the PHP 5.4.0 release can be found in the release announcement, and the full list of changes are available in the ChangeLog.
I'm going to upgrade my test server tonight. I always upgrade the test server to the first release and then upgrade live servers at the second point release.
Has anyone upgraded it yet and if you did does it break any of your sites?