According to a tweet from ESPN 980's Chris Russell, it would appear that this decision should not come as a surprise to either Rex Grossman or Donovan McNabb:
A source says both QBs were informed earlier this week, team informed today. Both QBs were told to not say a thing. #redskins
If true, it would appear that Mike Shanahan, Kyle Shanahan, and both quarterbacks were determined not to let this 'secret' out. But as of Wednesday morning, it did not appear that McNabb was aware of a decision, as he told the media he wouldn't appreciate being told he would be benched his late:
McNabb didn't appreciate that, as of Wednesday, he hadn't been told whether he was playing.
"I would have hoped," McNabb said at the time. "That's professionalism. Communication."