I recently updated six machines with pacman -Syu, including the 2.6.18 kernel. After rebooting, three machines booted off the previous kernel, e.g. 2.6.17, instead of the new one, which meant that the appropriate modules were not available. This was confirmed using uname -a. The kernel update had installed the 2.6.18 modules, and removed the previous modules.

All machines have a separate boot partition, using ext2 or ext3 filesystem. Booting is done with grub. Here is a typical menu.lst entry:

title Linux-2.6.18-md1
root (hd0,0)
kernel /vmlinuz26 root=/dev/md1 md=1,hda7,hdc7 ro
initrd /kernel26.img

Usually, only the title needs to be changed, and this was the case on all machines. We have been using mkinitcpio over a few updates with no problems.

A further investigation showed that /boot could not have been mounted when the kernel was updated, so the kernel files were installed on the root partition or raid, not on the boot partition. This was confirmed by unmounting /boot on all machines, and looking at the files in /boot. On affected machines, I could see the latest kernel image and associated files. Unfortunately, I did not check what was mounted before updating. However, /boot was automatically mounted on all machines after I had fixed the kernel problem and rebooted. I did not need to make any alterations to fstab.