I have a short Thermarest Prolite 4 and have also experienced the problem of the pad sliding around on the silnylon floor of my SMD Lunar Solo e (same thing with my old original full-length pad).
Although treating the tent floor is an option, I prefer to treat the bottom of the pad so it will stay put. The "slipperyness" of the floor is otherwise not an issue as long as the pad will not move around. Also, treating small areas of the pad will be much simpler and weigh much less than treating the larger tent floor.
Not finding any definitive answers here or elsewhere, I emailed cascadedesigns.com and, along with additional observations, posed this question:
"Can you recommend a treatment that will prevent slipping on silnylon, not be too tacky against other items such as clothing, and adhere to the pad surface permanently?"
The answer I received (very promptly, by the way) from customer service was:
"The best solution is to pickup some Seam Grip, a urethane seam sealer, from a local outfitter and place small dots of it on the bottom of the pad itself. This will help provide some friction for the pad to the tent floor without making the pad sticky."
Note they recommend a urethane-based sealer, not silicone. Hope this will help others.
I'm also posting to this recent thread "http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/7133/index.html?skip_to_post=51089#51089" that also deals with sliding pads.