Have tried setting a genkernel initramfs on my new amd64 install. Appending the "domdadm" for mdadm device scanning/assembling doesn't work (the md kernel modules aren't loaded). The problem seems to be a small typo on /usr/share/genkernel/x86_64/modules_load. We have as var name "MODULES_MDRAID". But we should have "MODULES_MDADM" like in /usr/share/genkernel/x86/modules_load. Have changed the var name to "MODULES_MDADM" and the new generated initramfs works for me. The problem seems to exist on all non-x86 arches (the var name is "MODULES_MDRAID" on all of them).
OK, I fixed this in SVN... thanks for pointing it out...
OK. This is resolved in genkernel 3.4.10, which is now in the tree and stable.