Yes, you can buy them and use them for purchases on the Japan iTunes Store. I’ve been buying mine from JBOX since last year. Someone on the AN forum suggested JBOX after I told them I was dying to get a song on iTunes but was presented with “This Apple ID is only valid for purchases in the US iTunes Store” notice. The iTunes page on JBOX will show you how to set up your account to redeem an iTunes card.
I recommend purchasing the ￥3,000 yen card since it costs about 8 dollars less than buying two ￥1,500 yen cards. JBOX also gives you the option of having your card’s redemption code e-mailed to you–a real time saver. These cards aren’t cheap so I recommend searching for your music through U.S. iTunes or Amazon first. I’ve found some great deals on Amazon. I buy used CDs from Amazon all the time.