With an office of 30+ users who run debian testing on their desktops, it's not a big surprise that any problems with debian testing can really come and bite us. Recently, a few developers who had been particularly slow with their upgrading hit a really bad circular dependency bug that basically stopped their upgrade cold in the water and prevented them from going any further. The bug in question is the initramfs-tools, kernel 2.6 and udev circular dependency.
The main problem is that udev requires a _running_ >= kernel 2.6.12 (soon to be >= kernel 2.6.15) to even be installed. It is not enough that you are just about to install the kernel. You must be running the latest kernel, which means the kernel must already be installed. The kernels on the other hand, depend on initramfs-tools .. which depends on udev. So udev will not install until you are running a kernel >= 2.6.12 but you cannot install those kernels unless udev is installed ... ouch.
Those who upgraded frequently enough hit that sweet spot when the latest debian kernel was 2.6.12 but did not require udev, so it could all be installed just fine. It did require a reboot after installing the kernel to install udev, as documented in the notes, but it was possible to continue. Those who took too long, or fixed their kernel to a particular version for various reasons eventually hit this bug when they did upgrade.
In the end, a few of the developers that were not quite so familiar with debian ended up reinstalling their system from scratch (debian testing install CD drops a >= 2.6.12 kernel in right away, avoiding the problem). There is a way to break this circular dependency without reinstalling though.
The 2.6.15 kernel (and possibly earlier versions as well. Did not check) does not absolutely require initramfs-tools. It is only the default option. Running dpkg -I on a kernel package shows:
Depends: module-init-tools (>= 0.9.13), initramfs-tools | yaird | linux-initramfs-tool
linux-initramfs-tools is a virtual package, so useless for us there. However, yaird is also an acceptable dependency. The solution then is to install yaird first, removing initramfs-tools, then install the rest of the mess (linux-image-2.6.15-1-x, udev).
Users of kernels that are too old may still be out of luck though, as hints given in the debian bug report suggests that even yaird requires a not too old 2.6 kernel.
Ah well. It is an unstable time again in debian testing, after the relative calm while sarge was being prepared for debian stable. There are quite a few circular dependencies now and people are reporting problems upgrading. In some cases, those upgrading from rather old debian sarge systems to the latest testing report that their desktop environments have become flaky (gnome and kde both). Switching to the other desktop, or purging/reinstalling those desktops seems to fix things.
Amazingly though, KDE 3.5.1 has made it into debian testing a mere 20 days (or less, I only noticed it today) after its official release. Certainly not the slow debian days anymore.