Now, this is kind of complicated to explain.
Both work just fine. Dual booting is done as intended, they both boot normally.
But, if I boot into Ubuntu and use it for any significant period of time, my next reboot into Windows will hard-reboot about 10 seconds after the OS "Welcome" screen appears. After this, I can boot into Windows normally and use it as long as I want, as long as I don't boot back into Ubuntu again, or this will occur again.
This only happens with Ubuntu based distros, like Linux Mint or Xubuntu. I tried with Debian, and it doesn't happen. It doesn't happen with <11.04, either.