Joseph Bafumi, Robert S. Erikson and Christopher Wlezien take a stab at it and come up with a likely net gain of 50 seats:
How many House seats will the Republicans gain in 2010? To answer this question, we have run 1,000 simulations of the 2010 House elections. The simulations are based on information from past elections going back to 1946. Our methodology replicates that for our ultimately successful forecast of the 2006 midterm. Two weeks before Election Day in 2006, we posted a prediction that the Democrats would gain 32 seats and recapture the House majority. The Democrats gained 30 seats in 2006. Our current forecast for 2010 shows that the Republicans are likely to regain the House majority.
Our preliminary 2010 forecast will appear (with other forecasts by political scientists) in the October issue of PS: Political Science. By our reckoning, the most likely scenario is a Republican majority in the neighborhood of 229 seats versus 206 for the Democrats for a 50-seat loss for the Democrats. Taking into account the uncertainty in our model, the Republicans have a 79% chance of winning the House.
In other words, a Republican majority is very likely and it will probably be a relatively narrow one.