Update 2005-09-27: I've unpinned all these packages. It turns out there was a package for 2.6.12 kernel, but they have changed the names used for the packages. For images, the package names start with "linux-image". For source the package names start with "linux-source". For headers, the package names start with "linux-headers". I've had some problems getting CUPS to work again, though, so I might try pinning gs-esp again...
Debian unstable has actually been a bit unstable lately. It normally isn't this way. I've had to pin a few packages to testing. Here is my /etc/apt/preferences:
Package: udev Pin: release a=testing Pin-Priority: 1001 Package: libtunepimp2 Pin: release a=testing Pin-Priority: 1001 Package: gs-esp Pin: release a=testing Pin-Priority: 1001
The libtunepimp2 in unstable was preventing Amarok from working. And gs-esp was preventing CUPS printing from working. The latest udev won't install without a 2.6.12 kernel, yet there is no package for a 2.6.12 kernel yet.