I've set up an arch installation on drive like this:
hda1: ext3 for /boot
hda2: LVM with / and /home (both on reiser4)
kernel used: kernel26beyond2-1

initramfs is generated properly with reiser4 module

booting dies with last error:
VFS: Can't find an ext2 filesystem on dev dm-1.

I've tracked it a little and found this in /lib/initramfs/init :

ROOT_FS=`/bin/fsck -NT $ROOT_DEV | awk -Ffsck. '{print $2}' | awk '{print $1}'`
mount -t $ROOT_FS -n -o ro $ROOT_DEV /new_root

i've made a simple test on other box (same kernel and reiser4 tools/libs installed):

[root@nevermore ~]# fsck -NT /dev/mapper/nevermore-home | awk -Ffsck. '{print $2}' | awk '{print $1}'
ext2

for now I used simple workaround:

if [ ! "$ROOT_FS" ]; then
ROOT_FS=`/bin/fsck -NT $ROOT_DEV | awk -Ffsck. '{print $2}' | awk '{print $1}'`
fi

and in /etc/mkinitramfs

ROOT_FS=reiser4