I've seen it in print. Sirius Black gets killed and falls behind the veil.
I've seen it on screen. Bellatrix Lestrange uses the Avada Kedevra curse to kill Sirius and he falls behind the veil.
She runs from scene, chanting "I killed Sirius Black" and escapes through the floo network.
And still I am not convinced Sirius is dead.
Part of me doesn't want to believe JK Rowling would kill of all of Harry Potter's father figures. First she kills off his real father, James Potter, quite famously and violently.
Then she kills of Sirius Black, best friend of James and godfather to Harry.
She kills Professor Dumbledore next.
It makes me think Arhur Weasley has little chance of surviving "The Deathly Hallows" if Rowling continues her pattern.
It also makes me wish Snape would become Harry's surrogate father just so he can be in line to die in a violent and painful manner.
The deaths of Harry's father figures is way too formulaic. For that reason I can't believe all of them are dead. I don't want to believe James is really dead. A part of me wants him to be an animagus, perhaps Crookshanks in disguise all this time.
That theory is unlikely. James is by all accounts dead. Sirius, he didn't exactly die convincingly. Something about slipping behind the veill makes his death a little suspect. It's almost like he was put in safekeeping, perhaps for his own good, by one of the aurors or even Dumbledore himself.
If I were a betting man, I would put my money on Sirius making a comeback in "The Deathly Hallows." Then again, it's a longshot and not a very smart bet.