I understand why you would want this, but I gotta vote against it. It costs money to produce the disks, packaging, and shipping to the stores. The money would be better spent on more development support, in my opinion.
I don't think people would understand. Install CD's are not benign, they have tools that will completely wipe a hard disk. Ubuntu could get a bad rep from doing something like this.
I think there's an inherent reason why it's a bad idea. People have gone accustomed to considering spam CDs for what they are, spam. I think it's important to make people realize that there is some value in it before pushing it on them. And replacing your OS is a major step, not like trying out crappy sharewares. Some media coverage and maybe intelligent advertising techniques are a better way to go.