Ezra Klein argues that because voters don't like Republicans either, 2010 might not be so bad for Democrats.
To be clear I don't think at this point that Democrats will lose either the House or the Senate next year, but here's an important fact to keep in mind:
In early September we found that a full 25% of the national electorate had a negative opinion of both Congressional Democrats and Congressional Republicans. But on the generic ballot voters falling into that category said they would vote for the GOP next year by a margin of 50-14. If voters are unhappy with everyone they'll generally vote for the party out of power.
If the economy is showing real signs of recovery by this time next year that individual households feel, then Democrats will probably have a fine election cycle. But if conditions are similar to today you're looking at some pretty big losses.