Mitsui Gold CD-Rs are said to be the best for long-term storage. They actually are manufactured with the intent of being stable, rather than to sell for $0 after rebate. (Yes, I've actually seen the $0 deal at Fry's on occasion.) They are more expensive--around $1.00-$1.50 each, depending on quantity.

You can find info about them by Googling "Mitsui Gold".