WordPress 3.4 Beta 1 is now available for download and testing.
What’s new in this beta?
Theme Customizer with Previewer
Flexible Custom Header Sizes
Selecting Custom Header and Background Images from Media Library
Better experience searching for and choosing a theme
New XML-RPC API for external and mobile applications
New API for registering theme support for custom headers and backgrounds
Performance improvements to WP_Query by splitting the query
Performance and API improvements when working with lists of installed themes
Support for installing child themes from the WordPress Themes Directory
Download WordPress 3.4 Beta 1