Today, Matt Mullenweg announced the availability of WordPress 3.4. WordPress includes significant improvements to the blogging application's theme theme customization, custom headers, Twitter embeds, and image captions.
More specifically with regards to theme customization, the new theme customizer allows you to play around with various looks and settings of your current theme and provides the ability to preview those settings without publishing the changes until you're ready to go public. For themes that support the theme customizer, you can also change colors, backgrounds, and image headers all from within the admin menu.
For the WordPress developer interested in code changes, many of the improvements in WordPress 3.4 are focused on XML-RPC, themes, custom header APIs, and significant performance improvements in WP_Query as well as the translation system. Developers will likely want to take a look at the WordPress 3.4 Field Guide for Developers for more of the details along with the how-to information they may need to know.
Attached below the fold is a WordPress provided video on the highlighting the change in this latest version of WordPress.