Nate links to a Congressional Quarterly list of ratings for 2010 congressional races and concludes that, although these listings give a sense of which races are more likely to be competitive, the CQ chart doesn't really say much about the chance that there will be a "wave" election that would switch partisan control to the Republicans.
The same day, Matthew Yglesias links to a recent Congressional Quarterly report entitled, "2010 House Outlook: Democrats Look Secure" and concludes that, yes, the Democrats look secure to keep their House and Senate majorities.
What should we believe? For the purpose of campaign strategy, you need to look at the races in each district, but to get a sense of what's going to happen overall, I think the best approach is to look at the national vote. There's lots of variation, but, overall, swings occur nationally.
Here's a graph I made after the election, showing the average Democratic share of the two-party vote for the House of Representatives and for president for the past sixty years:
From this picture, it looks possible but unlikely that there will be a 6% swing toward the Republicans (which is what it would take for them to bring their average district vote from 44% to 50%). Historically speaking, a 6% swing is a lot. The biggest shifts in the past few decades appear to be 1946-48, 1956-58, and 1972-74 (in favor of the Democrats) and 1964-66 and 1992-194 (for the Republicans). I don't know if any of these would quite be enough to swing the House majority. A more likely outcome, if the Republicans indeed improve in next year's election, is for them to make some gains but still be in the minority.
The other factor helping the Democrats is incumbency, which helps lock in a congressional majority (as it did for the Republicans after 1994) by bumping up the vote shares of the new congressmembers elected in swing districts. In 2008, John Kastellec, Jamie Chandler, and I estimated that the Republicans would need something like 51% of the average district vote to have an even shot of winning a majority of House seats.
P.S. Beyond the particular issue of forecasting the 2010 election, both pundits make interesting points. Nate discusses some factors that could help the Republicans (a continuing economic slump) and, on the other side, the Democratic Party's advantage in organization and fundraising. Yglesias suggests that, "given the contrast between ironclad discipline on the GOP side and the 'anything goes' attitude on the Democratic side, it looks like for a while yet we may be in a California-style dynamic where Republicans can't win elections but Democrats can't actually pass a governing agenda."