(a) The biggest and most convincing reason for me is this:

Sirius, the Dog star, goes through a 70 day period of invisibility. As far back as the Egyptians, this "death" of Sirius was a known phenomenon: the star they called Horus-Spd 'died', spent 70 days in the "Embalming House" and was reborn in the East.

We all know how JKR loves drawing from mythology and lore. My guess is that the Veil perhaps leads to an equivalent of an Embalming House, and our dear Sirius is going to be before 70 days are out. Mark my words: somewhere around the 1st of September, he's just going to turn up somewhere.

On a similar vein, there's this theory that "the Veil" is actually a metaphor for alien abductions in P. B. Shelley's poetry... but yeah, it does sound a little ridiculous to me.

(b) Sirius was not hit by Avada Kedavra; JKR always tells you when it's a *green* jet of light. It was probably a Stunning spell that hit him, and people don't die from being Stunned.

(c) What's Behind the Veil anyway? If it's any place the living can accidentally fall into (remember how Harry was drawn to go in there?), then if anyone can get out, it's Sirius. He got out of Azkaban; what could be more terrible than the Dementors?

(d) No one seems to be down about the whole thing... in sharp contrast to when Cedric Diggory died. Moody, Lupin, Tonks et al are almost in a *jolly* mood at the train station.

(e) Nobody really said Sirius was *dead* (yeah well, the notable exception being Dumbledore). People are just saying he's 'gone'. And there's Luna Lovegood's insistence that 'they'll turn up, they always do'. Now she may be one of those characters that get blown up in future books, but she had a pretty big role in this one, though with no discernable purpose. My guess is that *this* was her purpose... to tell you Sisius is going to be back.

(f) What's Sirius done so far? He's gotten out of Azkaban, that's all. He hasn't really *done* anything of consequence yet, and he's such a major character, it's impossible to believe his existence has been completely pointless. He needs to get his name cleared, he needs to live decently a few years at least, maybe even get together with that girl who was 'eyeing him hopefully' during his OWLs.. :D Come on, 12 years in Azkaban and 2 years in hiding - you call that a life?

(g) His death was *sooooooooooooo* easily preventable! If only:

(i) Dumbledore had explicitly warned Harry that Voldemort could plant visions in his head instead of cryptically advising him to learn Occlumency.

(ii) Harry had remembered the package Sirius had given him when he was trying to find out if Sirius was at home.

(iii) Dumbledore had told Harry about the prophecy, so that he'd know not to go and pick it up and have Death Eaters chase him.

(iv) *Someone* had done *something* while Bellatrix and Sirius were duelling, oblivious of Dumbledore's arrival... the Death Eaters were being rounded up by Dumbledore which left at least a couple of the Order members free - including Lupin!

(h) The death was so pointless, it makes you wonder if it happened at all. I mean, Harry's been through enough in the "Oh, I'm an orphan" area, right? There was simply no need to put him through it again.

(a) Well, Dumbledore says he's dead. Not once, many times. And according to Dumbledore himself, the dead can never come back. Well, we've never yet had a reason to doubt that he always knows best.

(b) JKR cried when she wrote it. Why would she cry if she knew Sirius was coming back?

(c) JKR *wanted* to make the point that deaths are usually pointless, easily preventable on hindsight and completely unfair. Sigh.

(d) "Behind the Veil" is widely recognised as a metaphor for death. The first I read of it was in Tennyson, actually, but the exact phrase may have been used at other times. Anyway, I looked it up, and here's the exact quote.. it's so apt! :(