Hi Rob,
"Does the fabric get a lot sweatier (or hot), does it cause the pack to shift more"?
Yes sweatier and no shifting. ;-)
Having the fabric directly against your back will keep your back warmer and wetter. While I have experienced this with my ultralight packs I haven't noticed that it causes shifting of the pack.
My pack has a sternum strap and hip belt. I pack it "burrito style" stuffing everything inside of my sleeping pad that is rolled into an open cylinder inside of my pack. This method forms a "virtual frame". The sternum strap and hip belt are probably the reasons why I don't notice any load shift.
There are some ultralight backpacks that have hip belts and lumbar panels that are constructed from 1/2" open cell foam with Cool-Max and open mesh back panels.
Check out the Katahdin and Pamola packs at Equinox.
http://www.equinoxltd.com/the-gear/backpacks-and-pack-covers/
Be advised that below is the original info copied and pasted from Equinox's website.
"Hip belt and lumbar panel are constructed from 1/2" open cell foam with a Cool-Max. Back panel is open mesh".
I tried to make the best sense of the italicized quote in my statement a couple of lines above the link that I provided. Email or call them if you are interested for clarification.
FWIW I am not sure how much drier your back will stay with either of these packs. You would probably need a pack liner to keep your gear waterproof and dry. There are pack covers available but the area between your back and the pack is still vulnerable to rain.
In the fall and winter having the pack fabric against my back actually helps keep me warm with less layers. YMMV.
Party On,
Newton