In GOF Crouch(as Moody) tells us that "there is only one known person to have survived the Avada Kedavra curse, Harry. Something Dumbledore says again, that exact phrase if memory serves, that Harry was "the only known person" to have survived the Killing Curse.
That phrase, keeps coming back to me "known person" That the only person to have survived the killing curse known to the general wizarding community.
Why does this mean anything?
Well, consider in OotP while Voldemort is throwing the Avada Kedavra around the Ministry at Dumbledore, what's Dumbledore doing? Walking sedately down the corridor talking to Voldemort. True he disapperates to avoid getting hit, and Fawkes takes one for the team, but Dumbledore didn't seek refuge from the curse.
Why?
There's also a line, during Draco's confrontation with Dumbledore. Dumbledore tells Draco "After a fashion, I came back" or words to that effect(I lent my copy of HBP to a friend so I can't check the exact quote)
Also, Harry(and so we)never actually SEES Dumbledore get up from the ground on "horcrux Island" Harry's fetching water and is attacked by the inferi and suddenly Dumbledore is on his feet driving back the inferi with fire they then get in the boat and leave.
Here's 2 theories each seperate, but based upon the text of all the novels to date it is supported.
1) Dumbledore survived the Avada Kedava at some point in the past and may(I said MAY) be to all intents and purposes immune to the deadly effects of the curse. This is unlikely tho because even if he is immune to the effects of the curse he's likely NOT immune to the effects of gravity.
2) Only PART of Dumbledore came back from the Horcrux Cave. As I said above, Harry never saw Dumbledore regain his feet, one second he's getting water the next he's surrounded by fire and Dumbledore is walking toward him.
In this case, I believe the potion in the basin that Dumbledore drank somehow split his body and part of Dumbledore is still lying in the cave. This is weakly supported by the line quoted above "after a fashion I came back" I believe that Dumbledore knew he wasn't whole and as we've seen Dumbledore is usually very precise when speaking.
Also, why was Dumbledore's portrait the only one sleeping?
The above are two of my pet theories on how Dumbledore might have survived. I don't necessarily believe that Dumbledore IS alive, just presenting two possible ways he might "come back"
Consider, with Dumbledore believed to be dead, he is much more powerful, able to work behind the scenes in secret. Voldemort thinks he's dead, so he drops his guard as Dumbledore works to find the horcruxes.