Fedora 18 was just released!

You can upgrade your current Fedora 17 like this:

# yum install fedup
# fedup-cli --network 18 --instrepo http://dl.fedoraproject.org/pub/fedora/linux/releases/18/Fedora/x86_64/os

Reboot and select Upgrade in grub menu

Enjoy!

Edit: now that mirrors are updated this should be enough:

# fedup-cli --network 18