Mungerville, you are right, book value is a moving target and is obviously different today than it was 2 months ago.
The point is however is that the element driving the book value down such as the long term put options value derived from various stock indexes contribute mightily to the book's apparent deterioration when they do not have a meaning in the context of short term emotionality of the market. ( As per Buffetts discussion on the Black Sholes formula and its implication on financial statements p 20 and his note 3 page 26); same can be said about the securities held in aggregate way below intrinsic value but temporarily devaluated by a depressive Mr. Market.
Book value is therefore significantly understated, Berkshire price today is near an artificially low book value and is therefore far below intrinsic value.
Anyway, what is remarkable is that operationally Berkshire did not have a bad year in 2008: to paraphrase Buffett the two most important businesses (insurance and utility) delivered outstanding results and have excellent prospects, on top of that capital allocation went also very well (apart from his confessed unforced errors with Conoco and the irish banks) and prospect look bright in this illiquid market.