Sharon Osbourne surprised NBC Television executives Tuesday with a Tweet revealing she wouldn't be returning for the eighth season of "America's Got Talent."  NBC officials claimed that the Tweet was the first they'd heard that Osbourne was even considering leaving, noting that she is under contract for several more years.  Osbourne's Tweet appeared to be in response to a message posted on the Twitter account for Howard Stern indicating he may not be back and that Osbourne wouldn't be back unless she got more money.  Osbourne replied that money is not the reason she won't be retuning, but didn't elaborate on what the reason might be.