I spent some time this weekend working on my http://dialthejoke.appspot.com/ application. The idea is certainly not original. I first learned about these systems when I saw Pirates of Silicon Valley. It accepts user submitted jokes and those jokes are then voted on. The joke with the most votes will be the played when you call 425-440-0035.
I built it with <http://www.twilio.com/ and Google App Engine. It's the first real app I built with twilio. It only uses the "say" verb so I still have a lot to learn about the twilio system. But I do have another app I plan on building during the evenings and weekends ahead. It's slightly more complicated but I think it will be a bit more amusing.
For those that are interested the source code can be found here: http://code.google.com/p/dialthejoke/.