Good news: PHP 5.4 is stable!
Some of the key new features include: traits, a shortened array syntax, a built-in webserver for testing purposes and more. PHP 5.4.0 significantly improves performance, memory footprint and fixes over 100 bugs.
Traits look very cool and the shortened array syntax is a very welcomed language feature. The built in web server is going to be quite handy too, especially for small hacks or perhaps even a small daemon that speaks HTTP.
I will be writing more about these features in the future as well as my experiences upgrading. For now, get yourself the source and happy hacking!