I've come across this "problem" while trying to run a Gentoo system image as a Xen VM on a Debian host (using a prebuilt xen kernel from debian). It appears that the debian initrd mounts /proc and /sys, but /sbin/rc wants to mount those itself in the sysinit stage (and dies if it can't, for whatever reason). It should check to see if they're already mounted before doing this.
Steps to Reproduce:
1. Copy a Gentoo system to a Debian Xen host
2. Set it up to use the debian xen kernel/initrd
3. Try to boot it
Init fails and prompts for root password for maintainance.
rc should see that /proc and /sys are already mounted, and go on about its business.
you didnt post `emerge --info` like the bug report page said to, so i'm going to assume you're using baselayout-1 since this should already be fixed in baselayout-2