two ways to look at it
just as the bond insurance industry is starting to fret about their AAA credit rating, a new player, backed by Berkshire's Billions, with lots of insurance experience (Geico and General RE) is about to enter the market with an AAAA credit rating.
if the current bond insurers ( MBIA & Ambac) lose their AAA credit rating we will see some real fireworks in the market with carryover to the financials, maybe Buffet is looking at certain financial, but by making this move he almost guarantees he'll get them cheaper in a few months.
or
consider His insurance companies (Geico and General RE) own a pretty large boatload of these mortgage instruments. This is simply a ploy to try keep these bonds on the books at face value. Ain't gonna happen as the foreclosure will keep getting worse and worse. This is only postponing the death of the major financial institutions