as far as I know if you install Windows 8 on the SAME computer you originally had W7 on (or even XP) where you activated Office 2010 - activation should cause NO problems --and won't count in the number of activations if you go back to W7 (or even XP).
Actually the strange thing is I never even got asked to activate office 2010 but it did ask me before installing it to enter the product key and I had to wait while this was verified before the install could continue.
If you DO run into problems activate by phone -- Ms won't give you ANY hassles if you are saying you are testing Office 2010 on Windows 8 development preview.
Enterprise versions (Office 2007) and terminal services product key for 2010 are legal multiple activations so won't cause a problem anyway.
Just posted this in case people want to try Office (2007 / 2010) on Windows 8 but are worried about losing activation status if they decide to go back to their original OS (XP / W7 ). I've left out VISTA as I don't think many can still be using this anymore.