I did an upgrade to F15 from my hybrid f14,15,16 and all went ok.
The upgrade hung 3 times on java docs, but eventually completed.
I had to monkey with the obsoletes and missing deps for F14 packages but with the newest yum the messages gave some clues and I deleted F14 stuff and installed.updated to F15 stuff.
Also running Virutalbox-4.1.0-beat3.
F13, F14 ok, now trying to fix an F15 that has a bad gdm screen with no mouse or keyboard action. May have to install again on that one.
F15 host runs fast and clean in gnone fall back mode.