I upgraded to Fedora 17 from 16 today using preupgrade, it seemed to go well apart from some issues with grub.
After the initial upgrade I couldn't shutdown the machine properly, it would through kernel panic. Some searching later and I find out that grub hadn't updated properly so I was booting from a Fedora 16 kernel :S
To fix this I ran grub2-mkconfig. This seemed to work but I was now getting errors telling me about a missing file and I had to press a key to continue but I could boot. I did some more searching and it turned out I needed to get grub reinstalled and the config setup correctly.
Here are the steps I used to fix the issue, these solved the problem for me but might not for you. They could stuff up your grub and you'd have to do a complete reinstall so use with caution if you aren't sure what you are doing.
Backup your existing grub2 config
$sudo cp /boot/grub2/grub.cfg ~
Reinstall grub onto the device. This seemed a little tricky until I realised the device I needed was the one with /boot on it, which for me was /dev/sda
$sudo grub2-install /dev/sda
Generate a new grub config
$sudo grub2-mkconfig -o /boot/grub2/grub.cfg
I'm now able to boot fine without any user intervention.
It's a pretty poor showing from Fedora to have this kind of issue, but these things will happen. As good as any QA process is things still can and will go wrong.