Interesting, while my suspicians are probably not going to be the most welcome things in the world, to me this sounds like a high profiling firing. Someone as well known and popular as Molyneux would be a PR disaster if they just tossed him, and the fiction of a "friendly parting" especially a sudden one like this seems fairly common in such cases throughout a variety of industries.
To be honest, while you can't help but like the guy, I've been a fairly harsh critic of him for a long period of time, and that includes his ever-growing reputation as a liar which I am not always willing to chalk up to him simply getting over-enthusiastic. I've oftentimes wondered if Peter was not a big part of the reason why series like Fable never actually reached the lofty heights that were originally promised.
At any rate, I will definatly have my eyes on him and where his new company goes. The telling sign is going to be how much funding he actually has behind this new venture, or if he's hoping to attract the attention of people to fund him. Something we can probably determine by how quickly we actually find something under development on a AAA level or if at all. If it takes a while, or his work is sub-AAA chances are he was booted as he didn't have the funding in place to make a high end transfer. If it's extremely quick and he almost immediatly jumps into a AAA-level project, then chances are he decided to leave himself after having obtained the resources and backing he needed for the move.
Any way it goes, we're liable to know the truth within 60-90 days. If he chose to leave on his own, chances are he had his next project planned out, and had secured funding, so we should at least hear about it and who he's in business with. Nobody "quits" without having their ducks in a row and being sure of themselves. If he goes dark for a while, at least as far as meaningful news of development goes, then we'll know he was canned and he's presumably trying to come up with something and secure funding having been tossed out while relatively unprepared for the occurance.
Did he surprise his bosses by walking out, or did his bosses surprise him by having him walked out? Time will tell.