On the insurance liability side, both AA and UA (rather than Massport or the Federal Government) were responsible for their own security at BOS. Therefore, if that security failed - resulting in the hijacks - there's a liability. Not sure what the situation was at IAD or EWR - but as the two aircraft that his the WTC originated in BOS those will be the primary claims.
As we all know, insurers will do their best to avoid paying out wherever possible - or at least minimise their exposure - and this will be one major area of contention. As wilbav8r rightly says, the colossal amounts involved are such that some sizeable firms could well go to the wall - and they will therefore have little to lose by keeping it in court and passing the liability onto the airlines.
And of course this being America - where people sue McDonalds when they spill coffee over themselves and win millions because it's hot; and where smokers are awarded billion dollar settlements - you can bet your bottom dollar that every ambulance chasing attorney in the country is drafting lawsuits by the thousand at this very moment.
On top of that, as with Pan Am after Lockerbie, there will be a massive loss of confidence in both carriers. That, coupled with their extremely high overheads, could well be enough to tip them over the edge. I suspect that AA are currently kicking themselves over their acquisition of TWA - although they are financially somewhat stronger than UA, there are few companies in the world that could pay the amounts we're looking at and survive.
Government intervention? Very possible - especially with a Texan President that favours big business.
This particular rumour came from someone I respect and who is well connected in the industry - we'll see what happens over the next few weeks!