Traditionally, the Linux kernel is software that I compile myself from pristine upstream sources for various reasons. I have three major kernel flavours that get built (server, desktop and notebook), and I am pretty current in running a bleeding edge kernel. This is not really necessary any more nowadays, but it's a tradition that works pretty well.
My kernels get built on sid and are packaged up with kernel-package, and equivs builds a dependency helper package which pulls in the kernel's dependencies such as initramfs-tools and takes care of cross-version updates like going from 2.6.29 to 2.6.30. Up to now, I was always able to run a kernel built this way on all my systems which can range from oldstable to unstable.
This has recently become a bit harder when unstable's udev started to complain that the sysfs layout was deprecated and that I should unset CONFIG_DEPRECATED_SYSFS in my kernel configuration or lose some of udev's functionality, since a kernel compiled without CONFIG_DEPRECATED_SYSFS doesn't properly boot on etch.
Since I wanted to avoid building even more kernel flavours, I decided to investigate which package I need to backport to etch to get kernels without CONFIG_DEPRECATED_SYSFS to work on etch. To my surprise, the cause for not booting on etch was not udev, but lvm2. With lvm2 2.02.39 (and the corresponding libdevmapper, 1.02.27-4) ported back from lenny to etch, my etch systems boot fine with CONFIG_DEPRECATED_SYSFS unset. udev can happily stay at etch's version, 0.105-4etch1.
From etch to lenny to squeeze, the lvm2 package has evolved quite a bit. Since there is only one LVM flavour left, the lvm-common package vanished in the transition from etch to lenny, and libdevmapper still has its own source package in lenny, but is built from lvm2's source package itself in squeeze. So, as a heads-up, there needs to be special handling for lvm-common when installing the backport on etch.
I decided to backport lvm2 from lenny to etch to allow this particular derivation from the distributions's versions to vanish in reasonably short time when my last etch boxes get finally updated to lenny. And my dependency helper has been extended to now pull in the backported version of lvm2 to avoid nasty reboot surprises.