Berkshire Hathaway is in a lot better shape than you think, although no one is happy that the economy is getting the stuffing knocked out of it, so it affects every company.
Berkshire does not have to post collateral (no margin calls) under any circumstances (including a downgrade--but to be clear the AAA rating on Berkshire is in no way in peril) on almost all of its derivatives contracts, Felix Salmon was apparently unaware of that, too. Capvandal explains it here:
http://capitalvandalism.blogspot.co...www.berkshirehathaway.com/letters/2008ltr.pdf
These positions are marked-to-market, but Berkshire Hathaway negotiated its contracts so that it does not put up cash in the intererim. It will only owe, if it owes anything, at the expiration date of the equity index put contracts in 2019 to 2028.
Operating income was actually pretty good, but net income reflected the fact that the mark-to-market went against Berkshire, even though (unlike AIG) there has been nothing going out the door for colllateral or margin calls. Even if there were, however, you can see from the financials, that Berkshire's cash position and revenue generation more than covers the entire nut. That is what we used to mean by AAA.
If there is a credit default, it will pay out on a credit derivative. This may be offset by the upfront premium. Tavakoli gives a good explanation of that in her new book:
http://www.amazon.com/Dear-Mr-Buffe...bs_sr_1?ie=UTF8&s=books&qid=1236522366&sr=8-1