Cannot log in as root directly, but su works

[Martetschlaeger Stefan]

Hi all, 

I'm currently working on an embedded arm system. I try to get it working with a read-only rootfs. I use an initrd to set things up, and do then switch_root into the read-only mounted rootfs which works quit fine but I cannot login as root directly on the login prompt. It says "Login incorrect" right after I enter "root".  I can, however, log in as any other user and then use su to gain root access. 
The weird thing is that it worked fine until 2 days ago as I replaced the /dev directory by a symlink to the mounted ramdisk I use for volatile data (/dev, /home, /tmp are also symlinks to the appropriate directories on this disk). I double checked that all device files are correctly set up on the ramdisk to rule out every obvious reason for any problem here. I can't see why that change should have broken something here. 
passwd, group and shadow are in place and contain entries for root. I get no messages in /var/log/messages after a login attempt as root. Also, the console I try to log in as root is listed in /etc/securetty .

I'm running linux-2.6.19.2, busybox-1.4.1 and Bash 3.2.0 as my default shell

Are there any more things to look at ?

Regards, 
Steve