I'm not sure if this will be of any help, but after a fresh install I tried to install kernel update 2.6.19-1.2895, since Fedora 6 had come out there had been prior kernel updates to this one and I kept getting dependency problems, mostly with mkinitrd & nash, and the dependency problems did not stop there, but I did. I decided to just update the kernel to prior version 2.6.18-1.2869 first as I had no dependency problems, then compile my modem drivers and do the rest of the updates including kernel 2.6.19-1.2895 on-line with yum.
I believe that once you go from 2.6.18 to 2.6.19 kernel is when mkinitrd and nash had to be upgraded also (and possibly more), the same may be true for you going down from 2.6.18 to 2.6.17 kernels. As it appears in the error you posted there is a mkinitrd problem, may have to downgrade it to a version compatible with 2.6.17 kernels. And a system.map file was not created "/boot/System.map-2.6.17.kernel-18.104.22.168/usr/src/linux", may be related.