This started out as a comment on Redbeard's Musings on Ubuntu, but grew into something worthy of being an article in itself. Enjoy!
As a Debian enthusiast and package maintainer, I feel qualified to comment. I am not one of the Debian elite, you'd have to pay me (or at least reimburse me) to go to a Debian conference, and I don't have the title "Debian Developer" which means not only do you maintain packages but you jump through hoops too.
Debian has a very rigid policy. All package maintainers have to maintain their packages in accordance with this policy or face the wrath of their peers. It is widely believed (and I share this belief) that the high quality of the Debian distribution is directly proportional to the adherence of packages to policy.
Now all of a sudden here's this upstart Ubuntu that builds on Debian. That's fine, and while there might be the occasional DD that feels jealousy that Ubuntu is building on Debian and getting all the credit, if you look around you realize there's dozens of projects built on Debian. It's a common thing, and encouraged. There's even infrastructure and packages in place to make it easier to roll your own Debian-based distro.
The irritation to the Debianistas is that Ubuntu doesn't always follow policy. They cut corners. They package nonfree software. They do all sorts of things that break policy. This is on the one hand annoying to the debian maintainers because they can't cut corners, but also because they can't just import the changes wholesale back into debian. They have to modify ubuntu's changes to fit policy. It's like someone not-quite-forking your project and sending you a deluge of patches that don't fit your coding style. You're thankful and you're annoyed. It's a love-hate thing. In some cases it causes real problems, not just annoyances.
So the Debianistas who think Debian is all that and a bag of chips naturally get annoyed when Ubuntu comes and does all this, which of course it does in the name of THE USERS and all that is high and mighty, but the Debianistas are perfectly happy with the state of usability in Debian, it rubs the wrong way. Ubuntistus gets mad because Debianistas are heartless elitist pigs, Debianistas get mad because Ubuntistus are busybody carelesss l33t hax0rs, secretly allied with the evil Gnome conspiracy to take away our user interface freedom.
It's not a matter of either Debianistas or Ubuntistus being right or wrong, or better or worse. It's just that common thing called differing viewpoints and priorities in close contact. The people wearing those shirts are immature socially-inept geeks. What's new here?
For the record, I like Ubuntu and although it has annoyed me on occasion (it annoyed me as a Debian user because it doesn't follow The Debian Way all the time), it has also made sharing Linux with others much easier and more worthwhile, and it has given Debian a kick in the pants which can only help. And I love the color scheme. Here's to a continuing symbiotic (if not entirely friendly) relationship between Debian and Ubuntu.