You know, in the infamous Black Hawk Down incident they shot down military aircraft with nothing more than an RPG. Devices that can put an explosive charge a few dozen feet into the air don't really require that much sophistication.
Black Hawk Down was an incredibly lucky (for the Somalians) accident that, if memory serves, was enabled by the combination of huge, close-by targets holding perfectly still, and a whole crap-load of rockets fired into the air.
All this is to say that you have about as much of a chance of shooting down a commercial airliner with an RPG as you do by throwing rocks at it.
With that said, good luck getting something with the firepower equivalent of an RPG anywhere close enough to a commercial aircraft to even be within the weapon's maximum range.
