I don't use plymouth.
Can fedup be used to go F17 -> F18
without having it used\installed.
Asking as it's was the last entry in a bz I had:
https://bugzilla.redhat.com/show_bug.cgi?id=880679
I think it doesn't really matter whether you use plymouth or not. FedUp downloads a
custom kernel+initrd to handle the upgrade. The error in your bugzilla seem like a broken
dependencies issue. That might really be an upstream broken repository, or it is a problem
with your local cache.