Q: I live in Australia. I want to buy an iTunes card in Australia and send it to a friend in the USA as a birthday gift. Will my friend be able to purchase iTunes in the USA with a card that was purchased in Australia?
A: Unfortunately, gift cards and gift certificates are only valid for the iTunes Store in the country in which they were purchased, so a gift card purchased in Australia will only be valid on the Australian iTunes Store. Likewise, even if you were to buy a gift certificate online through the Australian iTunes Store, this would be no more valid in the U.S. iTunes Store than a retail-purchased gift card would.
Your friend in the U.S. could sign up for an account on the Australian iTunes Store using your gift card, although this requires an Australian mailing address, and would technically be in violation of the iTunes Store’s Terms of Sale, which state that the iTunes Store in each country is only for the use of residents of that country.
The only really viable option in this case would be to either have another friend in the U.S. purchase the iTunes Gift Card on your behalf, or to purchase an iTunes Gift card from a third-party seller through a site such as eBay, where many U.S. iTunes Store gift cards are readily available for sale from third-party vendors. Although the Apple Online store does sell gift cards, they will definitely not ship them outside of the U.S. However you might be able to order a gift card through the Apple Online Store and have it shipped directly to your friend in the U.S.—you may want to contact the Apple Online Store for more details in that regard.