This is interesting. They have been operating iPredict since 2008, but apparently got a "no action" letter from the Commodity Futures Trading Commission in the US to allow US participants in the market (as long as they limited the markets in the same way Iowa University does for the IEM) 4 months ago.

The market isn't particularly efficient. For example, if you bought "No" on all the presidential candidates to win, it would cost $16.16, but would be worth at least $17 for a 5% gain. Of course, after paying the 10% fee on profits and 5% withdrawal fee you would be left with a loss, which is why this opportunity still exists.