Samsung has adopted a pretty good strategy with its Galaxy S model. Because USA is such a big market for them, they can afford to offer different versions of same phone for every single carrier. Well, the big 4 ones at least. This way people can still choose their carrier first and don't have to worry about a certain phone that is on another network, because they have their own versions of it on their network, too.
Samsung has already launched the Galaxy S internationally, Captivate version for AT&T, Vibrant for T-mobile, and Epic 4G for Sprint. All it's left is Fascinate for Verizon, which is coming very soon, and Verizon and Samsung have prepared a promo video to show it: