This post to the mailing list is all I know for now: http://www.redhat.com/archives/fedor.../msg01184.html
Apparently 4KSTACKS has now been removed.. but another issue introduced, I have not yet built this kernel.
A patch linux-2.6.5-nostack.patch
applied in the kernel SRPM needs to be removed (note that rebuilding the kernel-source rpm probably
will not deal with this). More when I try it out..