Bill told Forbes magazine in the early nineties (1994 as I recall) he would not stay at MS beyond 50. That he announced is not shattering news, nor a surprise. He is just on schedule.
As for not looking at SteveB, I would not read anything into that as well.
That the stock did not crash is also not a surprise. The market knows that Bills goal has been achieved; there is a computer in the home and on the desk. ('A computer on every desk and in every home running Microsoft software') The market also knows services is not Bills sweet spot, so again his timing and exit are right on point.
Bill should leave MS. The time is right. He has changed the world once, and with the BMG foundation, he has the chance to do it again. Compare that corporate behaviour to that found in Enron and WorldCom.
Go Bill go!