WordPress 3.1 will give us Post Formats. There is a lot of arguing on if themes and plugins should be able to add their own Post Formats. It is quite clear that the Post Formats serve a totally different need than everyone is thinking. In case you need to create your own Post Format, you should use a Custom Taxonomy instead of a Post Format. All of you still complaining about standardized Post Formats, should read Andrew Nacin’s excellent post On standardized Post Formats.
More information on the difference between Post Formats, Custom Taxonomies and Custom Post Types is to be found on Otto Wood’s blog: Post types and formats and taxonomies, oh my! and on Mark Jaquith’s blog: Post Formats vs. Custom Post Types.