The Senate looks very promising for the Democrats, not a single Democratic seat is a tossup seat--they are all Republican seats.
On the other hand, the House is going to be hard to defend. Although I think we'll hang onto our majority, albeit slightly reduced.
My predictions:
Senate:
60-64+ Dem
40-36- Rep
House:
230+ Dem
200- Rep
I think that's a bit to optimistic. There are several Democratic seats that, under the right conditions, would be competitive. Republicans are positioning themselves to hit Reid hard. Plus, Obama's cabinet choices have caused some major headaches for the Democrats on the Senate front. He's added two seats (NY and DE) for them to defend, one of which will be open. Not to mention that Bennet is an untested statewide candidate, Burris was appointed by a disgraced Democratic governor, and Gillibrand is likely to face a primary challenge from the left.
That's not to say the GOP doesn't have some major problems of its own with 4 open seats to defend, 3 of which are in swing states. Plus they have several vulnerable and/or unpopular incumbents who Democrats are already targeting.
All in all, Senate Democrats will probably have an easier time than Senate Republican in 2010 but I don't think we can go so far as to assert they won't be on the defensive in at least some states. I think Democrats are actually better positioned in the House, especially if Republicans continue to whip their members together on every single issue. I think Democrats could flip up to 20 more seats but I think they'll also be losing 10-15 they currently hold. If I had to make a prediction right now I'd go with something close to this:
Senate: 61D-39R
House: 264D-171R