While messing around with the widgets on my blog (http://nathan2055.wordpress.com/), I was able to create a "Powered by WordPress" widget you can add to a blog! Here are the steps:
1. Add an "Image" widget
2. (optional) If you want a title, put "WordPress" in the title box.
3. Put "WordPress" in the alternate text section
4. Add "Powered by WordPress" as a caption
5. Now, add http://wordpress.com/ in the link URL box if you use WordPress.com, or http://wordpress.org/ if you don't host with WordPress.com
6. Hit Save
7. Add the widget to the bottom of the sidebar (that's the end of the first section for me, might be different for you if you use two and/or three)
That's it! You now have a beautiful "Powered by WordPress" widget on your site! Please tell me what you think.