In contract law, you can always back out of a simple "X for Y" contract, even if it lacks such a clause or expressly prohibits it. If Microsoft gave Bethesda $900,000 for the exclusivity, Bethesda has the choice to refund that money and go back on their decision, just like if you gave a company $5,000 to redo your roof, but they found out they couldn't do it in a timely fashion. They don't have to follow up on the contract. They can refund you before they start regardless of what the text is. If you tried to sue on the grounds that "it says they have to." you'd lose. The only time you cannot back out of a contract is when something non-refundable is transferred by the affected party. For example, if Microsoft helped Beth do optimization and QA as part of the deal there is no way for them to legally back out. Just like recording artists cannot back out of recording contracts (helping make an artist a public figure can't be refunded or undone). Assuming the exclusivity contract is simple, Bethesda can, if they choose, return MS' funds and go back on their decision with no consequences. If MS objected, they wouldn't have a case in court unless they had very reasonable terms for backing out.
Now, the PS3 version is totally screwed with save bloat, stuttering, bad performance, etc. Patches aren't enough, and users without internet on their PS3s are totally screwed. As an apology, Beth needs to do something for them beyond simple patches. Whether that means backing out on their DLC exclusivity (if they can), offering the first DLC free, or something else they need to do something. If possible I think they need to can the exclusive content, even if it's time-limited.
It's unethical [censored] by both parties anyway. Someone needs to ninja this sort of thing into the Sherman Act for the good of the industry, but I don't see legislators caring about gamer problems any time soon. By the logic in the Sherman Act, paid exclusivity SHOULD be illegal in the States, as it is an abusive, anti-competitive practice. Sadly I don't see anyone taking anybody to court on it, and it's not an issue for here. Some extremely similar things are illegal though. IMO the law just hasn't caught up to tech yet. If this was any other industry there would be an uproar. Gamers are extremely tolerant of unfinished / broken products and unethical business practices compared to just about anyone else.
tl;dr: PS3 should get something special. Get on it Bethesda?
You need to step outside your little world of a box and come to reality.
They don't owe you anything, never mind offering services that would break pre-existing contracts. For someone that wants to think he knows much about legality and morals, you seem to ignore integral notions.
Seriously? Talking about taking companies to court over a game downloadable content? The world doesn't care about such trivial matters. What they DO care about is breaking contracts - and if Bethesda did so they would be breaking the law under their agreements with Microsoft about such exclusivity.
Please, get a grip of reality.