This has happened to me at least twice, possibly more, but it's not something I do often enough or on enough machines to file a bugzilla. So, I'd be curious if it's one of those Just Me(TM) things.
I install F13, minimal install, custom partition, no /boot. Then, if necessary, install fedora-rawhide-release, and upgrade to rawhide. Upon reboot, it fails. Either it can't find the root partition, or in one case, just brought me to the grub prompt, and reinstalling grub didn't fix the issue.
In every case, I've been able to easily fix it by changing the root=<uuid> to root=/dev/sd<whatever>. This has been on an Asus 1000HE, I haven't checked it on other machines. I'm just wondering if anyone else has seen this?