Using their traditional model for the generic preference between Democrats and Republicans, Gallup finds a 15% advantage for Republicans as of 2010 Nov 1. This result is outside the experience of Gallup polling in previous elections. However, using a linear fit between Gallup generic poll and outcomes for all midterm elections since 1950, we make the following prediction:
Republicans gain 69 seats in the House. The standard deviation on this estimate is 59 to 79 seats. That is, there is a 60% chance the final value will fall in this range. The 95% range is 39 to 99 seats.