Upgrading is no longer done via the installer. This was going to be mentioned in the release announcement, but, you know :)
For F18 fedup is now the One True Way to upgrade (well, you can still do it with yum, which as always is unsupported but in practice appears to work quite well). You run it from your current system and it downloads packages then reboots to trigger the upgrade, much like preupgrade did, but it's all new code that does not go through anaconda. Note F18 Beta is its first public release, so exercise caution.