After you remove domain from one account, you cannot add it to another one purchased with discount.
Also, read their definition of unlimited: http://dreamhost.com/unlimited.html
Total Bandwidth Provided: 13128 GB ($0.1/GB over) Total Used So Far: 1.362 GB Estimated Usage For Cycle: 2.484 GB Estimated Overage Charges: $0.00
Obviously that's not the "unlimited" plan.