Gmail2Blogger—Drag this to your browser's toobar or bookmarks!Thankfully, Gmail will post to New Blogger without a hitch (and you get a backup in your sent folder). You just have to create a secret Mail-to-Blogger address in Settings > Email. When you want to blog on the fly, click the Gmail2Blogger bookmarklet and address the post to your secret Mail-to-Blogger address.
Not quite as good as a shiny new BlogThis! but it will work for now. (And if you're a Windows user you can install the Google Toolbar—that has BlogThis! built in.)