WordPress 3.4 Beta 1 has been released. As always, this is still in development and not meant for use on live sites, but the team is planning for a final release in May if all goes as planned.
This beta includes a few notable additions over the current stable release, including a new theme customizer and previewer, the ability to select header and background images from the library, support for flexible custom header image sizes, and a much improved theme browser with the ability to install child themes from the official directory.
This is not yet meant for live sites, but please do take a few swings at it on a test installation. If you find any bugs, please report them, and contact the support forums if you need any help.