Both Kris and I are using it to keep our own personal, private notes -- it's our new default home page.
What I love about this use is the lack of context switching. I've been thinking about this a lot lately in the context of computer interaction and interruptions which flow through that space. Switching apps is disruptive, and the app I spend most of my time in is my web browser. With all my notes being only one tab a way, there is no transition into note taking mode or "to do list" mode.
And yes, of course I've been thinking of crazy ideas. TiddlyWiki is an interesting little secret right now. What if we glued it into Drupal? Create a page, and that page is a TiddlyWiki. Every page is an entire wiki, with TiddlyWiki's unique editing/reading style. I'm not quite sure what it would do, but I'm sure it would be interesting.
Oh, and in the best tradition of Steve Jobs, just one more thing: TiddlyWiki on your iPod...with wiki links to songs, playlists, artists, etc., of course.