You couldn't be more wrong. Many plastics convert into cyanide, ever smell burning/melted plastic? You're smelling cyanide.
No, that's not cyanide. A small number of plastics can produce a bit of cyanide, but most do not, and none do in quantity. Cyanide doesn't smell like burning plastic, either - cyanide smells like almonds.
Not that you don't get nasty stuff when heating plastics... i mean, PVC will release nasty PCB-like compounds, lots of stuff will release other chemicals (bisphenol-a from polycarbonate), and so on. Oh, and hydrochloric acid, some of the chlorinated polymers produce that in significant quantities too, though you have to be damned near burning them. The fumes from hot plastic are not something you want to breathe.
Clean aluminum is fine (safe) to smoke from. Some aluminum foil will smoke when heated (crap on the foil burning off), and aluminum foil that gets directly hit by flame usually doesn't fare too well.
Plastic should never be used in a smoking device except for as the chamber (water-filled part) of a bong, and only if the point where the bowl enters is below the water line (to keep the plastic cool).
Aluminum is fine, as is clay (not plastic-clay, real clay), wood*, stone*, and of course glass.
It's so cheap to get small glass pipes, and they taste so much better than even a really nice pipe of another material, i really don't understand why people still smoke non-glass pieces.
*Barring obvious exceptions, like a pipe carved out of realgar or asbestos (actually, this would be fine once it got filth'ed up with resin...), or a wooden bowl carved out of a particularly thick poison ivy stem (it can reach up to an inch in diameter easy, so this is theoretically possible).