I have a machine I still use which can't be upgraded beyond 10.2.8, no way, no how. It's the original iBook, the clam thing, that looks like a blueberry highlited accessory from a hair salon circa 1970... ( Heh. It's still cool, tho ) Now suppose that Xcode currently came with SDK's for 10.4 , 10.3, and 10.2.8 In that case, I would take very seriously trying to make my app back-compat to 10.2.8, because I would figure that alot of people were actually stuck at that plateau just like me-- that it was not just that they were too lazy or whatever to upgrade. I say all that so that you can tell where I'm coming from: are there similar, hardware intransigent version plateaus keeping alot of people using X.4 ? Or X.5 ? Or is it the case that everyone using X.4 simply hasn't upgraded, although they could? IOW, how many users would I be losing, *through no fault of their own*, if I target only X.6 or >= X.5 ?