These two companies started off insuring municipal bonds against default, then got in trouble with the complex mortgage instruments known as the Big Shitpile.
Is it? Certainly it's an exercise in transparency. Ackman found out a lot about what's insured by the companies, and made estimates of the losses he expects. (He figures about $12 billion on MBIA alone.) You can download his figures yourself and make your own estimate.
But this assumes he's got the right figures. MBIA calls it "half truths, innuendo and faulty analysis" aimed at giving him a "profit from his short positions." But they're not releasing financial code in response.
Which, again, is why Ackman called this an "open source model." It's aimed as much at giving him the political high ground as anything else. Here are my numbers, run them yourself, where are yours?