You guys think GTA and Call of Duty were always huge? They weren't. It took Rockstar and Infinity Ward a few tries before they got a breakout success. Now they're two of the biggest franchise in gaming.
With any luck, Uncharted 2 will be the GTA III/Call of Duty 4 of the Uncharted franchise (probably on a smaller scale) and the next game will have a huge week one. You can't expect those numbers coming after a modest success like the first Uncharted, though.