For a comprehensive overview of election projections, see 3 Blue Dudes. The consensus projection is Obama by 80-100 votes over McCain in the Electoral College, with a gain of 4-5 Senate seats (perhaps 55-45) and something like a 240-195 split in the House. (July 11, 2008).
This would leave the Democrats five votes short on cloture for limiting debate in the Senate, allowing Republicans to filibuster. There is an excellent chance of making up that difference in the mid-term elections, since only a third of the Senate is up for re-election in each cycle. Democrats gained in the Senate in 2006, and are likely to in 2008. The third and last cohort of Bush Republicans in the Senate will be up in 2010.