I don't know how I missed this, but Libervis.com has an overview article about the Debian and Ubuntu relationship.
Read the article here
There is a vision in my mind. It envisions a sphere of all debian based distributions sharing absolute and full compatibility with Debian base and each other in full respect. In this vision I can, for example, take Knoppix and use it to install packages from Mepis, Xandros, Ubuntu or any other Debian based distro that there is. The vice versa in all directions would apply too. It would be a perfect coherence and there would be no single piece of software that wouldn't have a .deb package you could install on anything that has Debian at its base.
Doesn't this sound good? It sure does, but there is a catch.