Remember Microsoft claimed that there would essentially be no difference between the to SKU's and that both versions of the 360 would be able to play all games right out of the box? Well, it looks like it didn't take very long before that promise was broken, because Sega plans to release, Football Manager 2006 (Soccer, not american football) which will require a hard drive in order to run. Check out the image of the case, look in the bottom left and you will see a tag that says hard drive required to run.
I guess on the bright side of things, I don't expect Football Manager 2006 to sell very well in , so I doubt this "hard drive only" business will not have any impact on gamers without a hard drive right now. However, I do think that this is a sign of things to come, games are inherently going to need storage and as games become more adavanced, the need for storage increases; the 360 has been out for less than six months and a title already needs a hard drive to play.
The best way to not have this problem is to not buy the core system.