theres an article that states that there is concern that bynum has a degenerative knee condition, meaning that his knees will never fully recover to previous levels. brandon roy has had similar issues with his knees, although it seems that bynum's condition is not as serious. the article goes on to say that he should be able to be a productive player in stretches, but may not be able to maintain his numbers from last season with the lakers over the course of a full season.
^ From the Yahoo forums.