$40 worth of change can net you $50 in iTunes. At participating machines, just cash in $40 or more worth of coins and select the iTunes option. The $10 iTunes promo code will be at the bottom of your receipt. Take your receipt home and enter the promo code and enter it at this page and fill in the info. Your $10 iTunes card will arrive by mail within 1-2 weeks.
This promo ends 3/6, though you'll have a bit longer to enter your promo code to claim your $10 bonus.