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It seems that the upgrade to Windows 8 is not going to impact any of your business process modeling needs1. Both CPN Tools (3.2.1 tested), (2.1.0 tested), and ProM (6.1 tested) runs with no problems on Windows 8.
I have only tested the 32 bit version, but I don’t anticipate any problems with the 64 bit version either. CPN Tools and Declare work with no problems on Windows 8 32 and 64 bit. ProM works with no issues on Windows 8 32 bit, but cannot be started using the executable on Windows 8 64 bit (which is the entry in the Start menu2.). ProM can be started using the .bat file on this architecture.
Atom ARM version has not been released yet (and I don’t have any hardware to test it on anyway), but according to rumors, it is very unlikely that CPN Tools will run on that version (it will not run Intel applications and CPN Tools cannot be recompiled for Atom ARM). Also, it seems that only Metro applications will run on tablets, so no process mining or declarative modeling goodness on those devises either. Note that this is based on my best guesses and rumors.
Edit: Changed Atom to ARM; thanks to Jens Holst3 Schumacher for the correction.
Edit 2: Added updated information about 64 bit versions.
Time person of the year 2006, Nobel Peace Prize winner 2012.