it seems to me the Anaconda installer doesn't do something quite right which leads to this random style failure. Its only random as to when it will do it, unless you do this, it will do it at some point.....
I've been doing Redhat for many years and never seen this problem before......anybody else seen this
you've obviously not been doing grub that many years :)
grub install on software mirror arrays is finally properly done in FC4 and therefore the future RHEL5.