If anything MS mis-calculated with XBOX, thinking Sony was the threat in 2000, but the real threat was Apple. Now it's 2010 and the money is not in the living room (and it never was really), it's in mobile.
Mobile wasn't exactly a hot industry 10 years ago. It's the job of a hundred billion dollar company like MS to make big investments in billion dollar markets, sure, but I don't think there's any denying Xbox was a miscalculation on MS's part.
See the market caps of Apple and MS now for lessons learned.