Just did a full upgrade up to current date of a SLES10SP4 (and OES2SP3)
32-bit virtual machine. After reboot, the machine did not come up.
Lateron I discovered that initrd did not contain any filesystem drivers
and this was because the etc/sysconfig/kernel file did not contain any
INITRD_MODULES variables. (In fact the first lines of the file
including the line with the INITRD_MODULES variable definition were
missing.) I am quite sure, that nobody including me did manually touch
that file - only installation routines have worked with that file.

As it seems to be quite common that upgrades break the boot sequence -
why is the boot sequence so prone to errors in Linux? You hardly ever
see those types of problems with Windows, Mac Os*, or Vmware ESX, ESXi.

And this one was quite annoying as I had to download the SLES10 SP4
media, and wait for the download to succeed to get access to the system
and repair the initrd content.
W. Prindl