The 2004.3-r1 x86 minimal live/install cd can't seem to do software raid1. I regularly used to boot up 2004.2, modprobe md1 and multipath to get this support, copy over my /etc/raidtab to /etc/ , then raidstart /dev/md0 through md2 with no problems. On the 2004.3-r1 livecd, doing the same and trying raidstart /dev/md0 gives this: cannot determine md version: no MD device file in /dev. This happens for md0, md1 and md1. Shot in the dark, something to do with udev being used instead of devfs?
Correction, "this happens for md0, md1 and md1" should be "md0, md1 and md2".
can-o-worms on #gentoo (who does not have a bugs account) is having the same problem with raid0. Gets the same error.
Read the release notes.