I know of two hurdles with pre-paid cards.
1) (already mentioned in other posts) You likely have to use the pre-paid vendor's on-line registration to associate contact information with the card. Most on-line credit card validation in the US only only uses the card info (#, expiration, cv2) and the postal (zip) code of the contact. If the pre-paid card has not been registered via the vendor's system, there is no zip attached to it.
2) Location of the payment processor. Epoch and many on-line payment processors actually process the transaction through international banks (outside the US). Most pre-paid card vendors don't allow their cards to be processed through foreign payment networks. So, if Epoch, netbilling, etc. are processing transactions through, say, Gibraltar, the transaction will be declined when the user attempts to make a purchase.
You either have to use a payment processor that uses US banks to process the payment, or you have to convert the anonymous pre-paid card into an actual pay-as-you-go pre-paid card that is linked to a real person. Most banks offering such services (converting a "gift" pre-paid card into a continuous-use, pay-as-you-go, card) support international payment processing for such cards.
There are likely other potential issues depending on the card issuer, payment processor, etc., but those are the major two of which I am aware.
Ttfn!