Warren Buffet’s Berkshire Hathaway is entering the bond insurance market and is likely to lure business from established rivals Ambac Financial Group Inc and MBIA Inc which are struggling with the credit market turmoil. Named Berkshire Hathaway Assurance Corp, the new insurer has received a license to operate from New York State’s insurance department and plans to expand the unit into California, Florida, Illinois, Texas and Puerto Rico. Municipal issuers finance such things as hospitals, road construction, schools, sewer systems and sports facilities. Berkshire Hathaway, which has a “triple-A” credit rating, expects the new unit to earn the same rating.