Blog.gears is an example of Blogger editor that uses Google Gears to make it work offline. You're able to create blog posts and edit one of the recent posts even if you don't have an Internet connection. As soon as you go back online, Blog.gears synchronizes the data stored offline with the data from Blogger's servers. There's nothing fancy about the editor (no rich-text editor, you can't add images or preview the post), but it's a cool preview of what we can expect from a new version of Blogger that will be offline-enabled.
A more advanced blog editor that works offline and it's not restricted to Blogger is Windows Live Writer. "Writer synchronizes drafts on your blog with changes you make when you're offline, so you don't have to worry about reconciling different versions."