And just for completeness, I just did a F17 -> F18 upgrade (32-bit) using fedup. Posting from the said system.
Worked as advertised.
Gotta say it looks like the devs bit off more than they could chew with F18. What with the delays, the new installer (and its limitations) - grub to a partition particularly in my case - and other rough edges, it just doesn't look a finished product.
May wait awhile before updating systems I really care about.