What archbishop_10K said.
Rest in peace means in the Peace of Christ - it means we hope that they died in a state of sanctifying grace.
When a Catholic dies and Mass is said for him, there is no proof he died in a state of sanctifying grace. We hope that he did, just as we hope that Heath Ledger and any other human being dies in a state of sanctifying grace.
Whether they were received into heaven or not, is the business of Christ alone. We still pray for them.
Anyway, the Church can only judge the intention of humans as far as they are exterior. And only to those whose exterior way of life reflected they were Roman Catholics and acted accordingly and were not pertinacious in mortal sin, e.g. the right for a public Requiem Mass is granted and the full ecclesiastical burial.
Despite the Novus Ordo actions, in the past Catholics who would not come to Church at least during Easter or otherwise regularly, and were known to leave away without good reason for such a long time, were not granted a full ecclesiastical burial nor a public Requiem Mass. Sometimes they were not even granted the right to be buried in bless earth.
Objectively Heath Ledger's situation was exteriorily non-Catholic and not in the state of grace.
Just consider: he was found by a "masseuse", with apparently a suicidal dose of drugs consumed, he played in immoral movies, and did not attend Mass monthly or yearly, and he openly approved homosexuality publicly. This means exteriorily judging, his soul's state was not alright. Nor could he get a Church funeral.
In normal times. The Church and world situation is so despairing and collapsed - during his entire life: he was post-conciliar, maybe not raised Catholic, never saw the real Holy Mass - that he subjectively may very well be excused.
Only God knows where Heath is now. Pray for him, that God granted him grace before death. Enough grace to attain heaven, through (long) purgatory. God is the only Judge. And by God I mean Our Lord Jesus Christ.
Let's imitate Mel Gibson. He remained respectful, mourned his death, and did not say other details or theological exposés to the press.
To me, this death of Ledger - who was an "example" for so many young people of hedonistic Western living today - is tragic and points out to me the necessity of living in the state of sanctifying grace, the necessity of prayer, of contemplating that death is always near and luring, that Judgement is harsh, and that young people are not spared per se.
I experienced the same as the father of a colleague of mine passed away unexpectedly. 50 years old. No more, no less.
For me personally, I hope to live through the Third World War, persecutions (I am an apocalyptic nutball I know) and further humiliation of the Holy Roman Catholic Church, to see the (temporary) Restoration of Holy Church and the (relatively short, but glorious) worldwide Era of Peace and conversion of people to Jesus Christ, before the final appearance of the Antichrist person and the Final Battles. I never lived before the Council (well, the world was not ideal then), and I would like to see e.g. our own parish church in which I was baptized (validly at least, by an old Dominican priest) in the 1980s, to be restored (high altar reinstalled, communion rails) and used for true Roman Catholic worship again, for our town to be converted from current sin and the omnipresent sinful lifestyles etc. etc. Maybe I am hoping for to much, and I know I should only hope on Heaven, as the glory of contemplating God, the All and Always Being, surpasses all nice and spiritual experiences on the world of course. But still I hope I will live to see it, if it pleases God. Though, if one is an Apocalyptic nutball like me who tends to take seriously certain Catholic visionaries and shrines' apparitions, I know that in Europe maybe as much as 5/6 of the entire human population may die or be massacred.
Let's prepare ourselves for death always. Not in a depressive or pessimistic way. I face future too, think of a carreer, vocation (family or religious life?), girlfriends, think about studying, world politics, economy, my parents, quite banal things (birthdays, partying), work, sleeping, sports, health, etc. etc.
And I am convinced that one can only be really content, happy, loving, satisfied if one is prepared for death and enjoys every day and week one is given by God on earth. Despite death, chastisements, diseases, mourning etc. There is much good. Let's enjoy life too. By having peace of mind and soul, by disciplining our soul ánd body (trying to do the latter a bit more now), being nice to others, enjoying social contacts, enjoying travelling (as far as possible). We are still spoilt and given many nice pleasureful things in our lives.
That is what I learn from this sad tragedy around the late Heath Ledger.
And please let us remember: Beati mortui qui in Domino moriuntur.