I saw a performance of Rocky Horror Friday night, and one of the players had awesome shoulder length dreads. I talked to him after the show and he said they were just extensions and his hair is actually short. He was actually wearing a poofy wig for the part, but when he took it off at the end his dreads looked completely natural. Granted this is kind of cheating and isn't really long hair, but for anyone that can't wait through the awkward stage this could be a good option. I imagine your hair only has to be 2-3 inches long to get it done.
More than you ever wanted to know about hair extensions.
They can also do damage to the hair you are trying to grow out because they are knotted or glued to your real hair (high on the shaft) and in the case of dreads can then add more weight on small numbers of hair causing them to pull at the scalp causing damage to the follicle and giving you headaches.
You are right, it's cheating and is probably better avoided if you are really trying to grow long hair.