Short list of what you can find there: Namespaces, __callstatic() magic method, accessing static members through $foo::myFunc(), fully rewritten ini-parser with .htaccess-like user defined ini files for CGI/FastCGI, improved OpenSSL extension, PCRE 7.4, and other fixes and improvements that will never get into 5_2 branch.

He recommends teching throughly before the major release to find all of the bug before the general public does. Builds for both source and Windows systems have been posted.