A few days ago, Mark Shuttleworth blogged about his view on the relationship between Ubuntu and Debian.
See the entry here
Im of the opinion that Ubuntu could not exist without Debian. So its absolutely my intention to see that Ubuntu is a constructive part of the broader Debian landscape. Its vital that Ubuntu help to sustain and grow Debian, because its the breadth and strength of Debian which make up the shoulders of greatness on which we in the Ubuntu community stand when we reach for the stars. That doesnt mean Im naive enough to think this relationship will ever be an easy one, but I would hope that the discussion doesnt turn into one of Ubuntu versus Debian. Because it isnt the case that one of them will succeed and the other fail. You could only perceive that as an outcome if you assumed that the two have exactly the same goals.