Hi there
remember that you CAN use W7 for 30 days before you have to activate so if you are just testing or comparing stuff between W7 and W8 you won't have to always activate W7.
In fact on a VM it's often better NOT to activate as you might want to test all sorts of configurations.
Hint -- depending on what you want to do -- the later releases of VMware flag the actual CPU and some "Real" hardware so moving a VM image between different HOSTS sometimes will cause the GUEST to request re-activation.
Earlier releases of VMware (rel 7/8 for workstation plus the equivalent free vmplayer releases) tended to use "more generic" virtual hardware which didn't flag reactivation so often.
Pros and cons for both issues -- later VMware releases are more efficient (can be up to 95% performance of running Native, have decent 3-D / DVD video and gaming) , older releases only had "generic video" which couldn't do gaming or DVD playing.
Your choce of course.
Windows 8 on a VM still wants a product key immediately and will probably attempt to activate as soon as connection to the internet is made.
Cheers
jimbo