Description of problem:
After "successfully" installing Red Hat 5.2 on IBM HS21 Blade with boot-from-iscsi disk, the kernel panics and says it cannot find the root device.
Meaning, the installation fails altough anaconda said it was successful.
Therefore, I wrote "successfully".

Version-Release number of selected component (if applicable):
Don't remember the exact version, it is the version that comes with Red Hat 5.2


How reproducible:
I have an IBM BladeCenter HS21, with a blade which is connected to a SAN by iSCSI. I configured the iSCSI targets and the iSCSI initiator in the boot order, and booted from the Red Hat installation media.

At the boot prompt, I typed in linux text mpath asknetwork, and entered an IP addresses when the anaconda asked me to. It broght up the network interface, and successfully connected to the iSCSI target. 
At first, I saw the disk from the SAN device as /dev/sda, and then I pressed F2 and configured the second and third targets (for mulitipath). Then, I saw the device as /dev/mapper/mpath0 and chose install Red Hat on it (default partitioning layout, minimal package selection). 
After a few minutes, the installation will finished without error, but it will not boot.
  
Actual results:
The system fails to boot - kernel panic - fails to find root device

Expected results:
Boot normally

Additional info:
A quick look at the GRUB configuration shows that there's no initrd configured to load. so I mounted the disk on another linux host and saw that there's no /boot/initrd-2.6.18-92.el5.img at all! so I ran the installation again, and before it rebooted, I checked at /mnt/sysimage/boot for initrd, and it was there- meaning anaconda failed to create the initrd.
So I chrooted to /mnt/sysimage, and ran /sbin/mkinitrd - it screemed it cannot find /sbin/iscsistart, so i exited from the chroot and copied /usr/sbin/iscsi* to /mnt/sysimage/sbin. Then, I chrooted back in, ran mkinitrd again and it successfully created mkinitrd. the command I ran:
# /sbin/mkinitd /boot/initrd-`uname -r`.img `uname-r`
Then, I added "initrd /boot/inird-2.6.18-92.el5.img" to /boot/grub/grub.conf and rebooted the machine. Then, the system booted just fine.

So, what seems to happen is that anaconda failed to create the inird because of missing files regarding iscsi.
Then, I tried installing again, on the same blade and iSCSI device, this time without multipathing. Meaning, I ran at boot prompt: linux text asknetwork, installed redhat with default partitioning on /dev/sda, with minimal software packages. The installation ended, rebooted the system, and it loaded just fine without me having to create initrd manually.
The only difference between these two installations was the multipathing part, which is weird because the mkinird failed at the first time because of iscsi-related files, which were used also in the non-multipathed installation.

I repeated these two installations on several other Blades and iSCSI devices, and got the same behaviour. All blades used were with most-recent versions of BIOS and NIC firmwares.