A couple months ago, I did something so small it doesn’t really deserve mention. Nial and I got OpenSearch working on WordPress.com for individual blogs:
Search of the entire domain has always been working, but this allows you to add a special search for one blog. To activate this, open Firefox (or Internet Explorer), and click on the search dropdown and you’ll see a new entry to “Add your blog name.” Select that.
Maybe I’ll add a plugin to WordPress with this code. I’m not too sure there’s a need though since there are already a couple OpenSearch plugins and this one only works in WPMU and PHP 5. There’s also a couple of WordPress.com-specific features like tags, privacy flags, and blavatar support in this one.