That's already the case with FaceTime and Photo Booth, which you can't use with the original iPad for a quite obvious reason.
I would be shocked if iOS 5 isn't available for iPad 1. An iPad is closer to a "real" computer than an iPhone, and the original iPad had way more powerful guts than older iPhones.
I'm also kinda thinking iOS 5 will work on the 3GS in some capacity because that phone will still be sold when iOS 5 is debuted. The iPhone 5/4S isn't expected to come out for a couple of months, so how much sense does it make to sell completely outdated phones? If this does happen and the next iPhone gets big storage bumps, perhaps Apple should sell 8GB versions of its current phone line for $99 and 32GB and 64GB versions at the $199 and $299 price points.
