As seen on the current news (6 may 2006), wrong kernelpackages may occur. Off course, the dev-team will fix those errors and mistakes as soon as possible, but maybe it's too late for you.
Actually, it was already too late for me, but here is the way to fix it. This page discribes how to fall back to a previous installed kernel version if the current one fails.
Start from the installation cd
The first step is booting the installation cd. When started, you type arch, like you would install arch.
Chroot to your normal root
When booted, you have a small, but working mini-linux environment with some basic tools. Now, you have to mount your normal root disk to /mnt.
# mount /dev/hdXY /mnt
Now, we will chroot to this disk, so you can use it like you booted your computer "normal". Off course, a lot will not work.
# chroot /mnt
Rollback to previous kernel version
If you keep your downloaded pacman packages, you now can easily rollback. If you didn't keep them, you has to find a way to get a previous kernel version on your system now.
Let's suppose you keep the previous versions. We will now install the last working.
# pacman -U /var/cache/pacman/pkg/kernel26-18.104.22.168-1.pkg.tar.gz
Maybe, you'll have to change the version numbers to the version that last worked for you, but it won't be a big difference.
Now the working kernel is reinstalled, you can safely reboot your pc, and boot you computer like usually. Don't forget to check the arch-newspage to check what went wrong with the kernelbuild.