In the 2008 election season the Intrade prediction market accurately forecast an Obama victory, with the final electoral prediction coming in at 364/174. The actual results were Obama-365 / McCain-174. Almost dead on, although Indiana and Missouri were reversed.
If Intrade is working as efficiently as it did in 2008, it looks like substantial gains for the GOP in the House and Senate.
On the Intrade market traders wager real money on the outcome of the 2010 midterms. Right now they are betting that the Republicans will pick up 8 Senate seats and at least 55 House seats. The House forecast has been a steady march upwards from 35 seats to 55. The Senate forecast has fluctuated from as low as +4 seats to as many as +9.
It looks highly unlikely that the GOP will reclaim both houses of Congress, but the House of Representatives is a lock according to the wagers on Intrade.
ElectoralMap.net tracks the data on Intrade and summarizes it into map format.