Is Samsung's Media Hub only available to new customers? Are old donkeys not allowed to eat carrots?!
Appears to currently be US only, and only added in the latest Epic 4G update. However the marketing materials keep mentioning Galaxy S devices, rather than any specific models, so it could well roll out to the rest of the Galaxy S range in future updates, or possibly even via the on-device Samsung Apps store that most Galaxy S's have (assuming your phone company hasn't removed it).

However it's worth bearing in mind that this is a transparent attempt to lock you into Samsung for future phone and TV purchases, as once you've built up a large library of paid content on their media store, you're probably not going to want to move to hardware where you can't watch that content. If you're happy with that then ok, personally I'll stick to more hardware independant media vendors.