You can contact Apple by phone to convert an Apple Store Gift Card to an iTunes Gift Code that is emailed to you. There is no ONLINE method.
Here are the steps that I just followed:
1. Call Apple at 1-800-MY-APPLE (1-800-676-2775) [in the U.S.]
2. Get connected to Gift Cards Sales
3. Provide the following info:
- Gift Card Number: 18-digit number under the bar code on the back of the gift card
- PIN code: 16-character code on the back of the gift card
- Your name
- email address
4. Stay on the call to confirm that you received the email with the subject line: "Your iTunes Gift Code" before hanging up
5. The email will have a 16-character code that you can use in iTunes
Cheers!
p.s. Though the webpage http://store.apple.com/us/accessories/giftcards/redeem states you can click on the "Apple Store Gift Card" tab in the Payment section, this option is actually NOT available when you try to purchase an iTunes Gift Card online.