It seems that everyone has forgotten how the last Uncharted sold, it was a game that spread by word of mouth, hardly anybody had it on their radar until it started booming.
Uncharted 2 has the advantage of being a very good sequel that got lots of advertisement, It would be a good guess that most the people who bought Uncharted 1 will have gone for Uncharted 2, to say we have surpassed the original so fast is a very good sign in my books, I have a suspicion that it will probably pass LBP, another steady seller.