Actually autodetect works, it's "do_md_run()" that finally fails. I have created the arrays with persistent superblocks, changed the partition type to 0xfd, and read all the bugs I could find on bugzilla relating to this. Everything works fine when booting, with autodetection saying All The Right Things, right up until... request_module[md-personality-4]: Root fs not mounted do_md_run() returned -22 unbind<hdd1,3> export_rdev(hdd1) unbind<hdc1,2> export_rdev(hdc1) unbind<hdb5,1> export_rdev(hdb5) unbind<hda1,0> export_rdev(hda1) md4 stopped. ... autorun DONE. It appears that md is compiled as a module in stock RH kernels, but for autodetect to work, it needs to be _in_ the kernel. (Notting has flatly stated in comments in other bugs that autorun is the only form of RAID RH supports under the current raidtools.) This can be remedied by compiling and installing the a correctly configured kernel, or perhaps by delving into mkinitrd. A more kludgey option is adding raidstart code to rc.sysinit. For everything to work properly, it has to go after modprobe, but before fsck of the non-root filesystems. This will not work for RAID on root. Also see related bug (don't have a number yet, since I haven't yet entered it into the system) relating to doing Software RAID installs.
The "other" bug is 4876, "Can't install RHL 6.0 onto SW RAID."
This is fixed in the latest mkinitrd in Raw Hide ; it adds the md-personality-* modules to the initrd.