Ok, I don't have one yet, My coding skills may or may not be up to actually making one. But I thought a prediction market in bitcoins would be a good idea. Because of their anonymous nature it would allow for a wider range of possible predictions than the currently available markets. My thought is a market in which anyone can offer something to be predicted, with the site admins taking a cut for judging which way it has gone. Things to be predicted could be submitted and then the judges would decide if they are able to judge that particular thing, and how much they would charge to do so, if they can't judge it it is rejected, if they can but the judge's fee is not met then the market is dissolved at the end and bids are returned. If they can judge it and the judge fee is met that is subtracted and the rest divided among those with correct predictions based on their market share.
First post here, fascinated by the discussion. This is a topic I've been interested in for many years, and it's great to read such excellent contributions.
I help run an open source prediction markets system at:http://news-guru.com/
It's what has become of the community based at the popular prediction markets site, "hubdub", after it announced it would no longer continue.
We're just starting out now, but have a few category owners, question makers and developers to help things come along, and things are progessing quite well.
We have some mathematicians and crypto people on board, so perhaps integrating some kind of bitcoin proof of concept could be a fun way to go!