Ok, I think we need to step back and clarify some things here. IMO, the hypothetical tail-shake mimicry displayed by some colubrid and non-rattled pit vipers is a very specific behavior- a high-frequency vibration only exhibited by the snake when it is in 'fight or flight' mode in response to a threat. While some forms of tail shaking have certainly evolved for other purposes (luring, for example), I don't think those are pertinent to this issue- though the question of whether the rattle originally evolved as a lure vs. a warning is certainly an interesting idea.
Rattlesnakes, other pitvipers, and some colubrids all vibrate their tails when under threat. I can envisage three possible evolutionary scenarios for how these groups came to do this. 1) tail vibration is a neutral trait ancestral to colubrids and vipers, and has been subsequently lost in other taxa for other reasons. 2) tail vibration automatically serves as a deterrent to potential predators regardless of mimicry (i.e. aposematism, which is why milksnakes outside of coral snake ranges still look like corals, though George Harper's work has shown it doesn't work so well there). 3) tail vibration is primarily a viperid adaptation to warn predators/tramplers away from danger, has been accentuated by the rattle in rattlesnakes, and is now mimicked by other rattle-less species for the same purposes.
The biggest problem I see is that audibly announcing one's presence, as some snakes are bound to do, is useless unless they have something with which to back it up. Snakes are incredibly fragile animals, and get eaten by just about everything larger than they are, if a predator gets the chance. Flight and camouflage are far more effective means of avoiding predation, and tail vibration would only seem to work if the animal was stuck where it couldn't (or wouldn't) escape, and was already targeted by a predator. If rattling evolved to warn quadrupeds away from trampling, again there has to be a reason to back it up, because what's it matter to a bison if it squishes a snake? Therefore, I really doubt that defensive tail vibration is either an ancestral trait or a general aposematism.
Hissing could be a highly ancestral trait- some lizards, turtles, gators, and birds all do it.
Nonvenomous species (esp. egg-eaters) also stridulate, like saw-scale vipers. Again, mimicry or aposematism?