This offer is good for flights before July 31, 2009 and is restricted to some fare classes. I registered just in case I get bumped and am rebooked into one of the higher fare classes. YMMV??
If you can find a cheap round trip, each regional upgrade certificate is worth about $100. The maximum you can earn is 6 regional upgrades for this promo.
Again, not all fare classes apply and you must register to be eligible.
Register here and search United.com for the eligibility rules. I cannot get them to come up at the present moment.