I would most definately say a myth. I go to a lot of my classes ripped and I learn a lot, especially the classes for my major. I smoke all the time and I have one of the highest GPAs for my major, so I say that that is a myth completely. I have read a lot of reasearch articles lately, and most say that the short term memory loss is present (not sure in what extent), but is completely reversible once the use has stopped.