Boot Camp: is it possible to restart directly to Windows from Mac OS X?
The fastest way is to use Bootchamp. It adds an option in the Mac menu bar where you can just click and choose "Restart in Windows". Q.E.D.
Alternatively:
$ sudo bless -mount "/Volumes/BOOTCAMP" -legacy -setBoot -nextonly;sudo shutdown -r now
(Edit: 10.9 requires sudo for shutdown, but this can also be used on previous versions.)