"At any rate if snakes are out chances are bugs are too what keeps bugs out will keep snakes out too. I'd suggest a bug proof bivy or bug net for bugs."
+1 on that. I would opt for a bivy with bug net or a bug bivy. That's a nice accessory if you tarp camp anyway.
Camping in established campgrounds may find you more pestered by rodents. I guess your could make a case for snakes being drawn to an area with a good rodent population. That should make you sleep better ;)
For that matter, if a snake was going to join you for warmth, a tarp wouldn't change the scenario and you haven't heard any stories about that here, right?
Cold blooded animals do need warmth to keep the chemistry working. Pit vipers have temperature sensitive sensors for hunting, let alone feeling warmth or cold (see http://en.wikipedia.org/wiki/Infrared_sensing_in_snakes). Reptiles will seek shade once they are too hot, so keep that in mind.
If you are scrambling in snake country, it's a good habit to check your hand holds for critters sunning themselves on top of a rock, log, ledge, etc. Losing your handhold could be as exciting as being bitten.