On Sat, 12 Dec 2009 15:00:23 -0800 (PST), infiniteMPG
wrote:

We found a product at local Lowe's Tecsun - Premium .25" Laminate
Underlayment which is a rather thick foam underlayment that not only
cushions the flooring, but acts as a sound barrier and also a moisture
barrier and appears that is could take up some slight variations in
the flooring. We're going to scope out the whole room and try and
assess what the whole picture is.

How hard is it to apply a leveler? We have heard some horror stories
about that, too, and would hate to put in a lot of work and end up
making the floor worse.

If the flooring is/can be glued on concrete (check product), you don't
need a leveler (all the time). Some adhesives have vapor barrier
qualities, just thicken up in the low spots. Gluing the flooring
eliminates the need for foam padding.

Sounds like you mean a "floating" floor with foam under it. That makes
sense on a second floor substrate. It reduces sound vibrations, etc.

Put a long level on the floor and rotate to see low spots. Pour the
leveler where needed.