Talk about putting a market into the deep freeze. MBIA Inc., the nation's largest bond insurer and one of the giants in the mortgage-backed securities market, said yesterday that it has suspended underwriting any new structured finance deals pending a possible split up of the company. In a letter to shareholders published late Monday, MBIA CEO Jay Brown wrote that the beleaguered monoline -- which saw its ratings affirmed earlier in the day by rating agency Standard & Poor's -- has "suspended the writing of all new structured finance business for approximately six months." It's unclear if other insurers will follow suit -- particularly Ambac, which also saw its ratings preserved yesterday by S&P -- but MBIA's decision clearly dampers any hope secondary market participants had for a short-term rebound in the MBS market. Brown also said that MBIA will eliminate its quarterly dividend in an effort to preserve capital going forward, and warned that more market-to-market activity lay ahead. "It is clear that the continued lack of any market valuations based on cash trades continues, and the extreme spread widening we and others are forced to rely on for valuation in the credit derivative market means we are likely to have another MTM [mark-to-market] in the first quarter," he wrote. "And the continued uncertainty surrounding the housing market, liquidity for refinancings, impact of the interest rate cuts and Congress' economic stimulus legislation mean we will need to continuously review our loss reserve modeling assumptions." MBIA remains concerned about Pershing Square investor Bill Ackman, whose short position and publicly-circulated research on the monoline have caused investor and market concern. Brown characterized Ackman as someone who "will stop at nothing to increase his already enormous personal profits as he systematically tries to destroy our franchise and our industry." For more information, visit http://www.mbia.com.