I experienced this error trying create the bootable ramdisk for 2.6.18-rc3. I noticed in menuconfig that RAID 4,5,6 are all together in one single option unlike 2.6.17. I assume because of this 2.6.17 generates raid4.ko, raid5.ko and raid6.ko, while 2.6.18-rc3 generates raid456.ko.

I just linked raid456.ko to raid5.ko so the mkinitrd would work, but I was hoping some of the maintainers would notice this post and fix it before the final release of 2.6.18.