The Itunes Store is restricted to certain countries and the content is not the same in the differnt countries it trades in. This is because of the deals Apple has with the record companies, tv companies and movie distributors. For instance the tv shows are only availble in the US store, not even the Canadian store has them, even though Canadians can tune in to US tv stations.
The way they make sure that you are in the country the store operates in is to link the store account to a credit card from that country.
There is a way around this, you can create an account without a credit card by buying a voucher. Another way is to create a .mac trial account (I think).
I think if you have an apple id, say with the zastore, you will will not be able buy on the store, since your address will be in SA.
Bummer.
Best way round it is to get a buddy to buy you a voucher in the US create an account that way.