When installing from an 0.4 iso to a SCSI disk, and selecting arch-initrd as default in the lilo.conf before installing the boot loader, the system barfs out a lot of insmod errors ("/lib/module/aic7xxx.o not found" et al) and fails to mount the root file system after reboot. After gunzipping and loop-mounting the /boot/initrd.img file from the freshly installed /boot partition, I noticed that the /lib/modules directory was utterly empty, which explains the errors.
No workaround possible except for manually creating a correct initrd, moving it into the /boot directory and rerunning lilo, all from a rescue system. Quite a hassle.

This problem does not persist in 0.5beta anymore due to the "different kernels" approach without initrd, but should probably be fixed in the 0.4 iso nevertheless if possible. The problem is a little different (and even worse) when doing an FTP install from the 0.4 version, as it does not support selecting a kernel and downloads the default IDE kernel image without even adding an initrd anywhere. In short: Don't do FTP installs anymore with the 0.4 installer if you've got to boot from a SCSI disk?
Workaround: snarf the kernel-scsi package manually, chroot into the /mnt dir, and replace the ide kernel with the scsi package using pacman as usual.