The Heath Ledger story seems unlikely to fade away in the near future, particularly as The Dark Knight does a roaring trade at the box office. It is also likely to continue as the buzz for the first posthumous Oscar since 1976 (Peter Finch for Network, if I am not mistaken) builds. The story will also continue as long as Mary-Kate Olsen maintains her silence, refuses to talk to investigators, and asks for immunity from prosecution if she does talk.
The facts that we do know is that the masseuse that found the unconscious or dead heath Ledger made calls to Mary-Kate before she called 911, we also know that the masseuse called again after paramedics arrived, and we know that Mary-Kate sent around her personal bodyguards. Mary-Kate's lawyer is maintaining that the starlet has told investigators all that she knows and therefore any further questioning is unwarranted and unnecessary.
It would seem that the investigators do not believe this and have gone so far as to obtain a subpoena. The subpoena by its very nature will compel Mary-Kate to testify before a grand jury if one is convened. The fact that investigators have gone to the trouble of getting subpoenas would suggest that a grand jury will be called. But law is a game in that sense and the threat of a grand jury is like shaking a tree and seeing what falls out.
It would seem that the investigators into the unnatural death of Ledger suspect that at least some of the drugs in the lethal cocktail of drugs that killed him were illegally obtained. The suggestion is that the prescriptions used were forged. The interest in Mary-Kate and the desire for investigators to follow-up with her could infer certain things but the allegations where these inferences are likely to lead have not been made public. Yet, most people should be able to join the dots on this one.
The claim for immunity immediately suggests that there is some degree of illegality involved that requires the maker of the statements, in this case Mary-Kate, needs protection from. I would have thought pleading the Fifth might have been an option in terms of not incriminating yourself at all. It is also interesting because Mary-Kate is the only person who investigators have wanted to speak to who has been reluctant to talk or who has claimed a need for immunity to be granted before speaking. Nevertheless, the other possibility is that all publicity is good publicity and drawing this thing out maximizes exposure.
There are drawbacks to maximum exposure like that the more coverage this gets the more it draws people out of the woodwork and then allows the investigators to put together a chronology of events even without Mary-Kate's testimony.
I enjoy watching the legal machinations unfold. After all it is my thing! My expertise is in this area.
Maybe there will be updates on this as things unfold or unravel, depending on which side of the fence you stand!
It is being reported that federal prosecutors have decided not to pursue the case of whether or not the prescriptions used by Heath Ledger to obtain the drugs that formed the lethal cocktail that killed him at this point in time. 
