I wonder if the DADA job is still cursed so that nobody will last more than a year? I want to know why Dumbledore, knowing this, allowed Snape to take the job. Anybody else get the impression that DD, even though he has little regard for Divination, has a somewhat omniscient view and allows certain events to take place? It could be argued that Snape finally got the job for plot purposes but it also seems like DD was waiting for the right time so other events could happen, like his death...I don't know...just a thought...
I have several ideas along this line. First, is that Snape knows his story to Voldemort (I'm only working at Hogwarts to spy on Dumbledore, really!) isn't going to work much longer. As we saw in the Spinner's End chapter, Voldemort has certainly noticed that Snape's behavior is ambiguous at best, and that his commitment to the evil side is not obvious.
And "Spinners End" is the perfect name for that chapter because it marks the end of the tale spun by Snape.
Dumbledore was seriously wounded destroying the ring horcrux. Perhaps it was a fatal wound, and Snape was merely able to postpone his death, but not prevent it.
Draco was given the "kill Dumbledore" assignment over the summer, when all these other events took place.
I believe that in the course of one summer (maybe even one month), three things happened: Snape feels pressure from Voldemort to be more open in his allegiance to V; Dumbledore sustains a fatal wound from the horcrux, but Snape is only able to delay his death, not prevent it; Draco gets the assignment to kill Dumbledore, and Snape learns of this plan.
Knowing all these things, Dumbledore decides to give Snape the DADA job. Dumbledore and Snape both know that if Dumbledore dies, Snape will not have an excuse to stay on as a teacher and spy for V - V will expect him to come and work for him openly. The DADA curse guarantees that Snape cannot return to Hogwarts. And Snape's vacancy as Potions master gives Dumbledore a chance to bring in Slughorn, whom Dumbledore needs because of the memory, and whom Dumbledore wants to protect from V. because of the memory
I don't think that during the summer Dumbledore anticipated that Snape would have to kill him. I think he (DD) expected to die from whatever gave him the blackened hand. That Draco would succeed and be in a position to kill DD came as a surprise to everyone, I think.
But Snape killing DD has a definite advantage in that Snape can plausibly tell Voldemort that he (Snape) must stay hidden and not work openly for V because he (Snape) has a huge bounty on his head as the killer of Dumbledore. This will allow Snape to work behind the scenes, hopefully sabotaging the Death Eaters from within.
Of course all this relies on you believing that Snape has been working for DD all along.