Based on current feedback, a slight revision has been made to the upcoming WordPress 3.1 release-related changes to the Theme Review Guidelines.
In short: in order to ensure UI consistency and full Theme-to-Theme portability of Post Format types for users switching among Themes in the repository, repository-hosted Themes will be required to add support for Post Formats, and to support all Post Format types.
Note that support does not mean the same thing as implement. Basically, repository-hosted Themes will need to add one line to the functions.php file, and that’s it. This one line will provide users with the Post Format UI (the metabox on the Edit Post screen). Any front-end Post Format-related changes to Themes remain entirely optional.