The biggest game of 2010 in America, for sure.
However, worldwide it will not be the top seller worldwide. It'll have a hell of a time competing with NSMB Wii, Wii Fit+, Wii Sports Resort, and Pokemon HG/SS. There's also a chance Nintendo pulls another 10miller out of its ass. WF+ and NSMB Wii anyone?
If we're only talking about home console games that release in the year 2010, currently its only competition is GT5 and CoD7, which Halo has a good shot at.
I think it makes little sense how people use ODST sales as a barometer for Reach's success. We may as well assume GT5 will sell poorly due to GT5:Prologue.