wow.. who would have ever thought that MJ would be good for memory..
There haven't been enough studies to actually prove this. Most show an issue while being high (and many studies combine marijuana usage with alcohol), but nothing that directly shows negative long-term effects. If this study is held up by their further human trials they mention, then it would indicate that pop-culture is probably wrong.
* Marijuana causes short-term short-term memory loss, but doesn't cause long-term short-term memory loss.
Unfortunately I can't find the study info now (@ work, so I'm kinda limited on spending too much time research pot...)