I don't know for sure, but---------------
I believe that FTM 2012 (or any program) that currently runs on Windows 7 will run on Windows 8 for two reasons.
1) I am looking to upgrade my computers also (currently running Windows XP). All of the documentation I have read states that, if a program runs on W7 it will run on W8. I believe the only difference between W7 and W8 is the horrible new interface. ( It has been bad enough to chase the drop down menus across the screen for the last several revision, but now Microsoft wants us to chase the windows themselves.)
2) If there was a problem of FTM 2012 running on W8 I am sure that Ancestry would not have made the decision of not to release a new version for 2013, but update FTM 2012