I've also got this problem, but I am using a ramdisk to boot. I have a swap partition at /dev/sda1 and a single large partition that is the rest at /dev/sda2. Running in VMWare.
grub.conf is as follows:
When I boot, the ramdisk loads, loads a bunch of modules, then I get the following error:
:: Scanning for blah ... blah loaded
>> Activating udev
>> Determining root device
!! The Root block device is unspecified or not detected.
Please specify a device to boot, or "shell" for a shell...
What am I doing wrong. I thought genkernel compiles everything in? So there should be a scsi module and the ramdisk should load it...