While I know this thread is mostly US users, I just want to note the difference between gift cards and e-cards for all other users.
Our online e-cards can only be redeemed for Premium--@mattjsrules is absolutely right.
However, in-store gift cards are also available in a few Spotify countries. These can be redeemed for either Unlimited or Premium.
You can click here to see if these gift cards are available in a store near you: http://www.spotify.com/gift-card/