And so it begins. We're about to witness the main event in financial institution internecine warefare: investment funds (MBS buyers) vs. banks (MBS sellers).
There have already been some opening skirmishes. The monoline bond insurers (MBIA, Syncora, FGIC, Ambac (and here), CIFG (and here), and--I haven't found any litigation with them on this, but there's gotta be some--ACA) have been litigating against some of the banks whose securitizations they insured for various fraud, negligent misrepresentation, and breach of warranty claims. Many of the Federal Home Loan Banks (Chicago, Indianapolis, Pittsburgh, San Francisco, Seattle, maybe others that I don't recall of the top of my head), which slurped up RMBS during the bubble, only to find them toxic, have brought (separate) suits mainly on securities fraud charges, but also on common law fraud and negligent misrepresentation claims. (See here for a totally dated, August 2010 estimation of the liabilities in these suits.)
Then last fall the financial world was shaken by the New York Fed, BlackRock, and PIMCO's demand letter to Bank of New York Mellon and Countrywide. That showed that A-list financial institutions were taking the range of problems with RMBS, from representation and warranty breaches to servicer malfeasance, seriously. (You can see the NY Fed, acting for the Maiden Lane LLCs, as really another representing AIG, essentially the mother of all monolines for these purposes.) But that wasn't litigation proper, just an angry growl, with a threat of litigation if things weren't resolved. (When you see the letterhead for the response, you'll see that BoA/CW is taking this mighty seriously. Despite the typo in that snippy letter, it didn't come cheap. These guys are lawyering up.)
And now we have the first A-list litigation. We have TIAA-CREF, New York Life, and Dexia suing Countrywide (and assorted other defendants). And it alleges invalid chain of title--the mortgage-backed securities are actually non-mortgage backed securities!