the death of sirius came as a huge shock to me and made me feel horribly depressed. however, most of what people are saying on this thread is true and i agree with it. the fact that the curse which hit sirius wasnt the cause of his death. If it had been the AV curse, I'm pretty positive that JKR would have elaborated on this, making it seem more connected to the other deaths that Harry has had to bear in his life. Almost everybody that was close to Harry that has died was killed by the AV curse: His parents, Cedric, many wizards that were important to the cause of fighting Volldemort... If Sirius had been killed by the AV curse, JKR would have said so. Also, regarding the arch and veil, I'm quite sure that it will play some huge part in the revisiting of Sirius in the future. The chamber was called the "Death Chamber", something that i did not pick up on until reading this thread. Someone said that it may be some kind wizard execution chamber, but i have something quite different in mind. Harry and Luna could both hear voices on the other side of the arch. When Harry is talking to Nearly Headless Nick, JKR writes in a huge emphasis about people becoming ghosts. Nick explains that he was afraid of death, and that was the reason that he had become a ghost, almost like it was an option that peolpe could choose at the time of their death. My theory is that the ministry may provide some way for people to become ghosts.......and i have some kind of rising suspicion that this arch is some kind of ministry tool to help people become ghosts by walking through it. This theory also has its opposition points, mainly stated by Nick, a ghost himself. He states something like, "Wizards can leave an imprint of themselves upon the earth, to walk palely where their living selves once trod, but very few wizards choose this path."
He goes on to say, "He will not come back, He will have.......Gone on."
However, he then later says, "I know nothing of the secrets of death, Harry, for I chose my feeble imitation of life instead. I believe learned wizards study it in the Department of Mysteries--" then Harry gets angry and tells Nick not to talk to him about that place. This suggests however that wizards within the misistry, within the department of secrets, study people becoming ghosts. Overall, I feel that this archway and veil have something to do with people becoming ghosts and that Sirius Might possibly come back as a ghost. This would we be both Joyful in Harry's situation, but I think it would overall be a more sad discovery. If Sirius did become a ghost, he would only be left with the pale and sorrowful imitation of life. He would then never be able to see the people he loved after he died. He would be able to be with Harry for a short while. But, after Harry eventually died(i know, i hate thinking about it too), Sirius would be left with no one. He would never see anyone that he loved ever again, including Harry. Therefore, unlike many other people, I hope that Sirius does not resort to coming back as a ghost, and that Harry will not be so selfish as to wish for this any longer. Just some thoughts that i hope will help people realize that Sirius's "death" may mean that it has to be the way it is, and couldnt be any other way.