There are two editions available, Desktop Edition and Server Edition. You may request any of these two. First you have to signup, then you can make your request.
Submit your request here
Update: Ubuntu has closed the free CD delivery program. The company website writes
"After delivering millions of Ubuntu CDs to millions of new users, our ShipIt programme has finally run its course. While we can no longer deliver free CDs through the programme, itís still easy to get Ubuntu. You can download Ubuntu for free from Ubuntu.com or you can buy a CD straight from the Canonical shop."