Global Crossing announced the fouth largest bankruptcy about two weeks ago. They had the same accounting firm, Anderson. The big accounting firms have gone from eight, to six, to four. Mary Sullivan from the DOJ released a report on the reasons why these mergers occur including: "across-the-board marginal cost reductions, marginal cost reductions for large clients, coordinated effects and unilateral anticompetitive effects".

Article on Global Crossing:
http://www.washingtonpost.com/ac2/wp-dyn?pagename=article&node=&contentId=A51536-2002Jan28
The DOJ report:
http://www.antitrust.org/cases/accounting/sullivan.htm