I don't think they're brains are big/complex enough. And if they are smart enough to have learned behaviour, or do tricks I would think of them more like cats than dogs, you know, is buisy sleeping, or not caring.
My turtle is slightly trained, but not to do tricks. I think that would take alot of patience, and time. Besides I think snakes don't have ears to hear like we do, so how could they respond to a command they can't really hear?
Just my thoughts.