I didn't know if it was a problem for you guys, but I ran across a forum that is using Vanilla without the links back in the left hand sidebar (and without the copyright at the bottom of the forum).
AFAIK, you are perfectly within your rights to remove the Vanilla copyright because the code is released under a GPL license. I could be wrong, though, and I am not sure what Mark thinks of it, but it is unenforceable.
I plan on removing the link back from mine (as we make it very clear that our site is based on Drupal and Vanilla in our about section) for sake of saving space in the panel (which is becoming very clogged) and general cleanliness.
I think the site is uncourtious for removing all mention of Vanilla, however.
Steorn still has the notice on the people/login pages--and are moving to some java-based forum/CMS (clearspace). I wonder if Vanilla 2 is going to use a more web-appropriate license that requires attribution and display of copyright notices--like MIT.
A copyright license can never require attribution on the end users; that would be a EULA. (I think the jury's still out on whether EULAs are even enforceable.) However, falsifying the attribution (like in the case where a web designer put a "powered by their-name" message) is clearly plagiarism and shouldn't be tolerated.
While the GPLv2 doesn't require attribution as-is, a copyright owner can modify it to say so (if they happen to like the other terms of the license).
I think there should be a few "powered by vanilla" buttons made, so we don't have to photochop our own from the images on this site. Text link is cool, but a graphic button looks much more attractive. Well that's what I've done anyway.