The article I linked to before at CityFile has the following update:

Update: Olbermann has issued an official statement on the matter: “That was my first opportunity to take even a long weekend to mourn my mother’s death and deal with the many sad logistics subsequent to her sudden passing. The source of this story is a liar and those who spread it without seeking confirmation or reputation are beneath contempt.”
Our Response: We were saddened to hear of Olbermann’s loss and found his tribute to his mother deeply moving. But if that was the reason Olbermann took time off two weeks later, we can’t imagine why Olbermann wouldn’t have simply said as much. Furthermore, we find it hard to believe one of his colleagues at MSNBC—a respected journalist, no less—would have attributed his absence to the “flu/allergy season” if Olbermann had made the perfectly understandable decision take a few days to mourn his mother’s passing.

While I think Cityfile’s skepticism is not unfounded, and the story Cityfile reported was hardly implausible, at the end of the day I think you have to give Olbermann the benefit of the doubt. My condolences on the death of his mother.