Ahh, well since the subject was brought up, my company sells flooring and mats designed just for these types or workouts so I hope you can forgive me if I throw a few shameless recommendations out there for you.  
If you want to do a full flooring solution and will be doing high impact workouts like the plyometric video in P90X, I would recommend our 
Plyorobic Flooring.  This is basically a rubber flooring solution but one thjat is less dense than normal to help absorb more shock.
There are also a variety of 
Gym / exercise mat and 
interlocking gym tile solutions available that can offer a temporary floor covering to help protect your normal sub floors and reduce sound transmission between floors.  
If anyone has any specific questions, please let me know and I would be happy to help.
One other thing, has anyone heard of these insanity videos?  I heard they make P90X look weak.  Considering P90X will kick most people's butts who are in good shape, those videos must be ridiculous.