Shares of the Santa Monica-based mortgage lender were down 20 percent this morning after the company postponed filing its fourth-quarter earnings. No reason was given for the delay, though Fremont plans to file an explanation with the Securities and Exchange Commission by the end of the week. It's certain to rattle investors because Fremont handles a lot of subprime loans - and we all know what a mess the subprime business has been these last few weeks. Here's the problem: During the housing boom subprime mortgages were given out willy-nilly to folks with lousy credit records or high debt. So now that the market has slumped and some of those subprimers are unable to either make payments or unload their property, delinquencies and defaults are up (what a surprise!) and the financial markets are freaking out. Another subprime outfit, Irvine-based New Century Financial, is restating three quarters worth of last year's earnings and reporting a loss for the fourth quarter. At last check, Fremont was recovering slightly, with the stock down 16 percent. Bloomberg News