Made the upgrade to Window 8 Pro this weekend. All things considered I have noticed a good speed increase over Windows 7. No major issues except when I plugged my quad into the USB port Mission Planner couldn't find it. So....
After a bit of digging I found this post: http://arduino.cc/forum/index.php/topic,94651.msg716650.html
in which I found a solution that I thought would be helpful to post here for others.
I'm running Mission Planner 1.2.14
Download the 3 files below
Double-click on the .cert file
Click "Install Certificate"
Select "Local Machine"
Select "Place all certificates in the following store" radio button
Select "Trusted Root Certificate Authorities"
Repeat the process, starting at step 2, replace step 8 with "Trusted Publishers"
Once you've added the .cert to both folders, right click on the arduino.inf file and choose "Install"
Open Mission Planner
This worked for me first try. Thanks to Louis @ Arduino for the great instructions, just thought I could save some folks a bit of time by relaying my direct experience with Mission Planner.