Interestingly at the last US presidential election there was a consistent and fairly massive arbitrage opportunity between US and European gambling exchanges.
US exchanges gave Romney a much higher implied winning percentage than the European exchanges did. The gap was so high that even taking into account currency conversion fees and the like there was several percentage points of guaranteed profit with deep liquidity that was available for weeks.
I you do a search for something like 'intrade betfair obama arbitrage' then you'll find plenty of site who were discussing it. The size of arb varied but it was persistently there for months varying from a couple of percentage points (that didn't make it worthwhile considering currency risk) to over 20% at one point.
Second question needs an answer. If there really was an arbitrage opportunity, it couldn't have lasted more than a few seconds. I doubt the "arbitrage opportunity" is taking into account all relevant factors.
If I remember correctly this was a few months before the election and since Intrade would hold the money in escrow the amount you would make off of interest was fairly close to the arbitrage opportunity
US exchanges gave Romney a much higher implied winning percentage than the European exchanges did. The gap was so high that even taking into account currency conversion fees and the like there was several percentage points of guaranteed profit with deep liquidity that was available for weeks.