This is pretty exciting news and I am very enthusiastic about it. The new WordPress Theme Directory has just been announced. It is available through http://wordpress.org/extend/themes just like the plugin directory and has all the features and infrastructure support of the WordPress Extend umbrella. The look of the plugin area has also been maintained to provide a familiar feel.
That being said, there are only a few themes on the new directory. You can upload your themes by just logging into the Theme Directory using your WordPress.org username and password and following the instructions and meeting the requirements on the upload page. The theme upload process is mostly automated except for a manual review. When you upload your theme, you should expect to receive an email (soon after uploading your theme, email reception times may vary) saying that your theme is waiting to be reviewed. If you do not receive this email, your upload did not work.
A lot of usable and popularly expected features have been added to the system including some that I believe the theme authors will love and will really help highlight them and their designs. Thanks to Joseph for his hard work on the Theme Directory.