I’m moving some old posts over to a new post type now that I’ve upgraded the sites at work to WordPress 3.0, and I needed a way to convert hundreds of posts in a category. The Post Type Switcher plugin does a nice job, but only on one post at a time. So, I wrote my own: Convert Post Types.
This is a utility for converting lots of posts or pages to a custom post type (or vice versa). You can limit the conversion to posts in a single category or children of specific page. You can also assign new taxonomy terms, which will be added to the posts’ existing terms.