If you're like me, it takes a while for new ideas to finally break their way into my brain. I've seen several demos on TDD and it makes sense, but the demos generally seem trivial. Brett L. Schuchert has a video on the Shunting Yard Algorithm (converting normal math equations to RPN - think HP Calculators without the Equals key). His 4 part video series consisting of 6 videos covers a reasonable problem, and presents a good application of TDD.
http://vimeo.com/10941784