The discount code is Buythedomain and only applies to new registrations of com/net/org/info/biz/mobi.
I just now used the code to buy 10 domains @ $5.99 each. I tried to buy 12 domains, but the discount was only applied to the first 10 in the list, so I removed the other 2 from the cart. I may try again tomorrow to buy the other 2 domains, just in case the limit is 10 per day. I have no idea if that is the case. It may be just 10 total, period.
I also have no idea how long this promo code is good, so get 'em while you can.