It seems that some prepaid visa/mastercard gift cards do work, and others don't. For the ones that don't, the funds end up being held for 7 days, and there isn't a thing the gift card issuers can do about it.
Regardless, with any of them, you have to register your billing address first, and if unsure if a given card will work or not, only go for adding $5 to the eshop account, so that only that amount will be tied up if it doesn't work.
For those that are in Canada, the Bank of Montreal prepaid travel mastercard does work. (Its a reloadable prepaid card.). The CIBC advantage debit card however, currently does not, due to the fact that ALL canadian merchants have to specifically participate in order to process as credit rather than debit. (The CIBC advantage debit card however does work for Wii/DSi reloading, since that goes through the american credit card processor.) I don't know the specific brand name of the prepaid visa off hand, that I tried, that failed, but its website, as listed on the back of the card, to register the billing address was www.giftcardstore.ca/visacard/.