The recession was worse than we thought. It started earlier, and from a lower peak than we previously calculated. The Economist has this chart showing how bad we thought it was, and how revisions to the data show that it was worse:
The revised report shows that the recession started in the 4th quarter of 2007, more than a year before George Bush left office. As for the depth of the plunge, it's the worst most of us have ever seen:
Economic activity now appears to have declined by 4.1% from peak to trough, easily outstripping the 3.7% dip observed in the downturn of 1957-58 (in 1981-82 the drop was just under 3%). The decline in real GDP in 2009 was far and away the worst annual performance since 1946, when America was dismantling its wartime economy.