WordPress.org has announced the beta version of WordPress 3.4. You are advised to use this version for testing purposes only. You can see more about how to report bugs you find, etc., on the WordPress announcement page.
According to WordPress, these are some of the major changes you will notice:
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
And these are some of the “under-the-hood changes”:
New XML-RPC API for external and mobile applications
New API for registering theme support for custom headers and backgrounds
