As I was doing updates yesterday I ran into some serious problem. How this happened:
Well I did an apt-get upgrade which first of all broke my nvidia driver. I also had messages like: udev: missing sysfs features; please update the kernel or disable the kernel's
CONFIG_SYSFS_DEPRECATED option; udev may fail to work correctly.
So I did a dist-upgrade to fix the nvidia problem and the sysfs message. This upgraded to kernel 2.6.30-2-686 and broke my system.
During the boot debian was unable to find my lvm volumes. Quite annoying as my /usr is also an lvm partition. First I though I had a corrupt filesystem.

I downloaded the latest system rescue cd from http://www.sysresccd.org. In the rescue prompt all seemed fine, I could mount my lvm's without any problem. So it wasn't a problem with the partitions. I booted again in my broken install and then saw that there were no entries under /dev/mapper. I checked and the dmsetup packages wasn't installed. So I installed it running apt-get install dmsetup in a chrooted environment with the rescue cd.
apt-get install dmsetup
Then the sh*t began, dmsetup was in conflict with devicekit-disks. No problem I though I'll just continue and apt-get will delete the needed packages. But I wasn't paying attention (was already late). It removed all the gnome libraries which meant that my gnome programs were also uninstalled (revelation, virt-manager, ...).
Luckily I don't use gnome as window manager. But this didn't solve my problem, still unable to find the lvm's. I chrooted again in my rescue environment. And found out that there were no lvm commands available.
apt-get install lvm2
This solved my problem, and I was able to boot again. Maybe a lesson to be learned was not to blindly upgrade when you are in sid.
So watch out when doing a dist-upgrade on debian sid. Especially if you have gnome!

I found some info on http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=550434 .