The following is a post from a different site, though one I found to have a great amount of information. ALMOST as much good information as here!
--------------------begin quoted text------------------
The only carpet pad I recommend these days is the Might Gripper available at
your local Home Depot store. You only need small boarders pieces of this pad
to keep even large area carpets from slipping, truly amazing stuff. My son
has it on his hardwood floor (that I refinished) for 2 years now with no
signs of deterioration. But again wait a month before using even this pad.
I don't use carpet pads under my wool area rugs, there seems to be no need
in my case, the furniture keeps them in place. During the first year or two
be sure to shift the carpet a few inches in each direction every month or
so, to prevent a "aging border" from showing up in the finish.
Yours Truly, Joseph the Wood Floor Doctor
-------------end quoted text-----------------------