GDM greeter fails to start after F17->F18 fedup update

I'm suffering from one of the various X won't start problems after updating to F18, I can get it running from a tty and then use startx but that doesn't work for my own userID only for root. On the X console the last thing I usually see is the plymouth-quit-wait message and a spinning cursor that sits there for ever more (hours at least).

I've been round lots of threads and to be honest my head is so full of stuff that I'd be hard pressed to detail anything right now. I started with this late yesterday afternoon so I'm about 24 hours in to trying to fix it, at least 10 hours elapsed at the keyboard.

I suspect that there is now something wrong with the plymouth theme, it seems that the initrd rebuild sets the wrong name for /boot/initrd-{version}.img when it should be naming it initramfs-{version}.img. That's fairly irrelevant now, I see someone has put in a BZ entry to point out that dracut -f doesn't do the right thing but it might be why I'm currently seeing a text boot up despite using the rhgb kernel parameter.

If anyone can help with debugging this I'd be appreciative. Sorry for the lack of explanation, if you tell me what files/info you want to see then I'll supply it.

Cheers

Brian