I don’t have time to write a full blog on this myself, but here are some of the best commentaries to explain what’s going on that I’ve seen recently.
On AIG:
http://www.nytimes.com/2008/09/16/opinion/16lewitt.html
On Lehman, counterparty risk, and the dangers of a derivatives meltdown (which an AIG collapse might well precipitate:
http://www.moneyandmarkets.com/Issues.aspx?NewsletterEntryId=2234