In the US for a shareholder suit to succeed on the basis of fraudulent business practices (including violation of securities laws) the facts must be "cogent and compelling" in proving that the company had an intent to defraud investors. The is probably going to rule out any shareholder suits since I haven't read anything to indicate allegations that BSC was "cooking the books" like Enron and Worldcom were. Marking to market and valuing mortgage securities is a tough call these days. What is selling is going at fire sale prices because of the overall market jitters in that sector. And high quality securities are being trashed along with junk. Shareholders might try to block the sale to force a higher price but it is likely that it will go through anyhow. 30% of all shares are employee owned - they won't try to block the sale as their jobs would definitely be lost if the bank went under. The SEC has said nothing and if they had any intentions of looking into BSC's accounting practices, I think they would have spoken up by now.
JP Morgan got a bargain. One of the reasons for the bargain price though is that they did not have time to do a proper job of due diligence on the acquisition so they are offering a low price. A couple of analysts I've read have mentioned that although BSC was the largest underwriter in the mortgage bond business before their collapse, that segment is basically worthless now. What JPM is interested in is the prime brokerage business. Another interesting item I read is that when BSC put out feelers on the sale the only two who showed up were JPM and a private equity firm. Since the private equity firm would never have gotten government support, JPM won the bid by default. JPM does still have some risk but its pretty well hedged by the fed guaranteeing the potentially riskiest investments.
This thinking is borne out by the markets behavior today. As a group financial stocks are down significantly as there is some speculation over who will fail next. JP Morgan is up significantly. Still way off its 52-week high but up quite a bit from Friday.
Edited to add - I stand corrected on litigation - JP Morgan evidently does anticipate some litigation. I did find one article in a UK newspaper talking about it when I went looking
http://www.guardian.co.uk/feedarticle?id=7391831
Evidently shareholders are looking for lawyers willing to file lawsuits - the question then becomes will the suits go anywhere?