Ugh, it's a compatibility problem.
"Beginning with Linux* kernel version 2.6.18*, the dmraid* utility 1.0.0-rc15 supports RAID 0, RAID 1, and RAID 10.
Beginning with Linux kernel version 2.6.27*, the mdadm* utility 3.0 supports RAID 0, RAID 1, RAID 10, and RAID 5."
I believe that Ubuntu 8.04 uses a 2.6.24 kernel, dmraid 1.0.0-rc14 and mdadm 2.6.3
I can't find a package list for Fedora 11 that actually has the version #s of the packages it includes, but 11 does have the 2.6.29 kernel so hopefully dmraid and mdadm should be upgraded accordingly.