If you upgrade your Fedora 15/16 to Fedora 17 before it is officially released, there is a high probability that you encounter a bug, where your computer receives a kernel panic when you try to shut down/reboot. This is documented in bug 820351. The problem is that since Fedora 17 updates repository is still not open, your system might still have an older kernel installed. To fix this issue, just install the latest kernel from Fedora 17 updates-testing repository, like this:
$ yum update kernel\* --enablerepo=updates-testing
After one more reboot, your system should be fixed.
Please note: This bug should not appear once we officially release Fedora 17 (a.k.a GOLD), so that extra step won’t be necessary.
EDIT: The problem persists even in final release and different steps are required.