In addition to "major" bankruptcies (e.g., those firms with assets greater than $50M) tracked by Bankruptcydata, the U.S. government tracks all bankruptcy filings by type (e.g., Chapter 7, Chapter 11, Chapter 13). You can find detailed bankruptcy statistics at the U.S. Courts website. For the twelve months ending June 2008, there had been 33,822 business filings versus 23,889 for the same period in 2007 (a 42% increase).
The chart above presents annual (calendar year) business bankruptcy filings from 1990-2007 (click on the picture for a larger image). Interestingly, the number of filings had been trending steadily downward from 1990 through 2006. In 2007 the numbers started to reverse course. I expect that trend to continue in 2008 and 2009, with total business bankruptcies reaching 45,000 in 2008 and 55,000 in 2009. I will update these data when the final 2008 numbers are released, which should be sometime in March.