Lately, I've been using LVM2 on all my Linux boxen. I don't know if this problem is LVM-related, but after installing Debian "squeeze" on the HP Proliant ML350 G4, it refused to boot up.
The GRUB screen would come up, and the kernel would begin loading, but it would fail to find the root filesystem on the LVM partition (/dev/mapper/. . .)
The helpful message told me this could have one of three causes - (1) wrong root partition (unlikely), (2) need to add rootdelay=, and (3) module not being loaded. At first, I thought it was case #3, but it turned out to be #2.
I had trouble finding out what exactly to write after "rootdelay=". First, I tried rootdelay=1500. However, this resulted in a seemingly endless delay. Then I tried rootdelay=90. This resulted in a somewhat less interminable delay, after which the boot process finished. This was the first time the ML350 G4 had booted from a SCSI disk - HURRAH!
Anyway, the delay still seemed too long, and I began to suspect that the number must be in seconds. So I tried rootdelay=5 and, sure enough, there was a 5-second delay and the system booted just fine.