Measurements: Upon hitting the Shutdown button (Windows) or typing in "sudo poweroff" (Gentoo/Fedora/Ubunt), I hit start on the stop watch.
When the system is totally powered off, I hit the stop button on the stop watch.
Margin of error: 1 second, either way (but more likely higher than lower).
Gentoo: 8 seconds
Fedora 18: 11 seconds
Ubuntu 12.10: 12 seconds
Windows 7: 22 seconds
Windows 8: N/A (Not available for me, yet)
wins the shutdown race. This is probably due to the fact that it only
runs what I need it to run, whereas Fedora and Ubuntu come with various
error reporting tools (that may not be what adds 3-4 seconds).
Fedora and Ubuntu are tied, as 1 second is my error margin.
Windows 7 comes in last, at 22 seconds. This may be due to the "drivers" and "software" that were installed by the Fujitsu DeskUpdate tool, but that does get me close(r) to the OEM state.
Windows 8 is unknown, as I do not have access to the RTM version yet (maybe my school will upload it to their network software servers soonish?).
Just an FYI, my university is breaking any agreements (that I know of) when they upload Windows installation media to their servers, as they make every student purchase an Undergraduate bundle which includes access to the "latest" Microsoft software (specifically Office and Windows).