You can now import and export your entire blog and comments as an XML file that WordPress.com will generate for you. You’ll find the export and import options
under “Manage” now under “Tools” in your blog admin area. The XML format is an extended version of RSS 2.0, and it will be built into the next downloadable release of WordPress (3.1).
We hope you guys really like this feature, but go easy on it! It’s very resource intensive to dump all the content of a blog out, so only do an export if you really need it. We do on-site and off-site backups hourly and daily, so your data is already very safe against service failures.
The export is mainly useful for moving content between WordPress blogs that you run, whether it’s moving from your self-hosted blog to the convenience of WordPress.com or the other direction. It includes posts, pages, comments, drafts, private posts, categories, and more. We don’t yet have a plugin that can generate this format for older version of WP, but it’ll be coming soon and when it’s done we’ll post about it.