In a service provider environment it is often required by customers to support old server installations for much longer than it is recommended by vendors.
Recently I had to move some Xen VPSes running Debian 3.1 to a modern Xen host running lenny and the 2.6.26 kernel. Since Xen is a FPOS and has been unable for years to maintain a stable ABI, I always use in the domUs the same kernel version of the dom0. The Debian Xen kernels require using an initramfs to load the paravirtualized block device driver and mount the root, at least. This would usually not be a big deal, but in this case breaks the VPS because the lenny version of udev is started in the initramfs and the system has either no udev or the old sarge version. The most immediately visible effect is that /dev/pts does not exist, which makes ssh less than useful. This is my solution:
dpkg --purge hotplug udev printf '#!/bin/sh\nmkdir /dev/pts\n' > /etc/rcS.d/S02mkdir chmod +x /etc/rcS.d/S02mkdir