Investment caps (as maintained by the Iowa Electronic Markets and the UBC Election Stock Market) level the trading opportunities among traders. Whether investment caps help with prediction accuracy has not yet been determined conclusively. However, without an investment cap, commercial election stock markets may be dominated by a small number of traders. These are single traders making unproportional bets compared to the average trader. The existence of transaction costs for investing and trading in commercial election stock markets may also reduce their efficiency. IEM has no transaction costs.
Update: Latest Electoral Map from Intrade shows Obama ahead again in the electoral and popular votes.
http://electoralmap.net/index.php