Is there any reason you couldn't just sign up for another account? Different credit card? Not the same as getting another year for $10, but you'd have unlimited bandwidth and disk for a year for ... $10.
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.