I want to know why it's not recommended to have a mattress made
entirely of temperature-sensitive memory foam. Cost is an aspect but
many manufacturers say that it's because people would find it too soft
and you would sink all the way down.

I thought that temperature-sensitive memory foam softens at body
contact but deeper down, were the body heat doesn't reach, it remains
hard so that it provides good support. That would mean that a mattress
made entirely of memory foam (the temperature sensitive type) would
get soft at the top, in the first 2 inches maybe, but the rest of the
mattress would remain hard. How can that be too soft? How can you sink
all the way down in something like that?

I'd appreciate some clarification on this.

I have an expensive memory foam mattress. About $900, if I remember
correctly.  It works great, very comfy, no back pain.  I had only
minor back pain from some mattresses, but this is very good in that
category.  The foam gets more and more indented as the night
progresses.  It is slightly awkward to roll over or move to a place
that isn't indented. I am about 25 pounds overweight and I have to do
some slightly vigourous movement in the middle of the night to roll
over.  I am sure that it would not work to have an all-foam mattress,
it would just get softer and softer at the night progresses. 
Insulation values (for example, due to the thickness of the foam, as
suggested in your letter) don't stop heat transfer, they just slow it
down and the night is long.

I have tried a memory foam mattress with 8 inches of memory foam and
it does not provide enough support. The memory foam its self does not
seem to be as firm as you might think when looking at it and as a
matter of fact it can be very soft when holding it between your
fingers. When you lay on it theough the memory foam actually seems
buoyant. I purchased a Cloud9 Celebrity mattress for 2000.00 less than
a Tempur-pedic from www.wholesale-memory-foam.com and it was very
comfortable. It was really unique because of the customizable base
layers that you could switch around. The cusotmer service department
really helped me choose the right one.

That website address was http://www.wholesale-memory-foam.com/

We are a memory foam mattress manufacturer and would like to answer
this question for you.  Memory Foam was desinged to be temperature
sensitive and weight sensitive.  It reacts to a body by its level of
body heat and weight.  An entirely memory foam mattress would work! 
The density would have to be very high in a 5 to 6 lb. range.  The
problem with it is that most companies do not create a mattress dense
enough to support a person on its own.  The reason no company
manufactures this mattress is due to cost constraints and the fact
that the perfect memory foam mattress has a base.  A memory foam
mattress with a base gives the best support by supporting the memory
foam itself.  If you had a 10" memory foam mattress with no base, you
would not hit the bottom of the mattress as the memory foam can only
compress so much.  A supportive base leads to a higher level of
comfort for you - the consumer.  Trust us when we say that you will be
better off using a memory foam mattress with a base as that design has
been tested to show it would provide the most comfortable surface for
you to sleep on.  In the meantime you can visit us at
http://www.memoryfoamfurniture.com for a ton of information about
memory foam.

Thanks,
Ross