systemd: no device ever becomes pluggable

[Thomas Sailer]

I've upgraded a few machines to Fedora 17.

One of them does not boot anymore. No device ever becomes plugged, thus
systemd eventually times out waiting for the disk device
(dev-sda1.device) and drops into the emergency shell.

The device is there and accessible;
udevadm trigger --type=devices --action==add does generate add events
(as seen by udevmonitor)

Strangely, though, none of the devices have a sysfs path; also, I only
have the two disk devices mentioned in fstab, while on working machines
I have many more.

Can somebody give me a hint on what goes wrong, or on how to debug this
further?

Thanks,
Tom