[squarejawhero]

Since every game is coded differently, Microsoft has to look at each backward compatible Xbox game individually. It has to test each one and create a game nugget to store on Xbox 360 to enable backward compatibility. This is a long and arduous process, and it's forced Microsoft to prioritize some games over others. Thus, "the best-selling games" will come first, and others will come after. Also, Microsoft is trying to get third-party publishers to include code nuggets in this fall's games so those will be instantly backward compatible from day one. Either way you look at it, if you want the Xbox 360 to play Xbox games, you'll want the hard drive, and a little patience from Microsoft on this one.

http://xbox360.ign.com/articles/663/663045p4.html
IGN.com