I've read how many successful Oehler style PSP structures use earth, poly, carpet.

My question is how are these floors with bare feet in the wintertime?  Do they seem too cold?

I was thinking about floor insulation using natural materials like straw.  If the floor was earth, poly, straw, poly, carpet; then the straw would be protected and might last for decades...

I also think radiant floor heating might work far better in Oehler structures than in structures with a concrete slab.  One inch diameter poly pipe costs about $.25/ft.  You could use 1 foot of poly pipe per square foot of floor.  It would not take too much work to bury 1" diameter pipe as shallowly as possible and level the earth on top of it. 

I would terminate the floor pipes in easily accessible places with good valves.  That way the design could connect to any heat source, solar, wood, compost, geothermal, whatever.  If the house was split level, I might give each level its own separate radiator coils.  I would gladly accept the sight of poly hoses running along the interior of the house, in order to permit flexible complexity.

I think the polyethylene pipe would be prone to collapse under foot once it had warm water or oil running through it.  maybe not, though.  at any rate, I like the idea.

hmm yes running extremely hot liquids could be a problem. 

It doesn't hurt the poly pipe (much) to be walked on.  To improve the strength and uniformity of the floor an inch or two of earth will make it feel solid.

I also think radiant floor heating might work far better in Oehler structures than in structures with a concrete slab.  One inch diameter poly pipe costs about $.25/ft.  You could use 1 foot of poly pipe per square foot of floor.  It would not take too much work to bury 1" diameter pipe as shallowly as possible and level the earth on top of it.

If you install a hydronic in-floor heating system (like pumping hot water or glycol through 1" poly) it would be terribly wasteful to install the pipes directly in contact with the soil beneath the floor. The earth is a giant heat sink and will likely steal all the heat energy from your pipes and then some.
PEX (cross-linked polyethelyne) tubing is the most commonly used material for these systems nowadays because of it's durability and it's ability to withstand high temperatures. I recently saw 1/2" pex for sale at a local hardware store for $0.60/foot. A concrete slab is not neccesary, but the tubes absolutely should be insulated from the earth.