If you buy movies or tunes etc. it will go to your giftcard credit but if you buy an app, it will go on your Visa. I found that the hard way, I used a 50$ gift card, bought like 20$ worth of app and it got billed to my Visa instead of using my 50$ credit. I now have to use it for movies, tunes or other. I didn't know you couldn't use it for apps so it almost feel like I got ripped off, but it was my fault, it's in the fine prints...
Just had to jump in on this, there are multiple blogs and such on this topic, trying to allow us to use to cards for apps, so we can only hope (for Canadians anyways).
im in canada and i found a way to use gift cards go to your account and change the address to a US address to a relatives or a post office box address then redeem your gift card