If you run a big site or many blogs and would like to create some automated exports or run into memory/runtime issues with the web based exporter then this wrapper around the existing web-based WordPress exporter ( http://core.trac.wordpress.org/browser/trunk/wp-admin/includes/export.php ) will likely help you out here.

Grab the file from https://github.com/tott/WordPress-CLI-Exporter and copy it to your WordPress root folder or any other convenient location. 

Make sure to set $wordpress_root_dir and $_SERVER['HTTP_HOST'] according to your environment. 

Then execute the script with the following parameters ( brackets indicate optional parameters ):

php cli-exporter.php --blog=blogname --path=/tmp/ --user=admin [--start_date=2011-01-01] [--end_date=2011-12-31] [--post_type=post] [--author=admin] [-- * category=Uncategorized] [--post_status=publish] [--skip_comments=1] [--file_item_count=1000]

blogname: the name or blog_id of the blog you like to export
path: the full path to the directory where exports should be stored
user: the WordPress username or ID that will run this export
start_date: only export posts older than this date (format: YYYY-MM-DD)
end_date: only export posts newer than this date (format: YYYY-MM-DD)
post_type: export only posts matching this post_type
author: export only posts for this user_login or user_id
category: export only posts with this category name
post_status: export only posts with a particular post status
skip_comments: if this is set to 1 then comments will not be exported
file_item_count: how many items to include per file (will automatically break up into several files at the specified interval)

I hope this is useful. Feedback welcome at http://wp.me/p1lkCl-43Q