Hmm, I once visited a website called something like
www.dumbledoreisnotdead.com, and the author had some good arguments. One I remember clear is this: We've encountered Avada Kadavra some times now. Fake-Moody on a spider, Wormtail on Cedric... It is described as a green flash, and then the targeted person just falls over. However, when Snape uses it, Dumbledore is
flung from the tower. The actual force sends him flying. Why? The author's guess? Since non verbal spells played a pretty large part in 6, chances are that while Snape said Avada, he thought something else. All smoke and mirrors. It would also fit the phoenix associations coming from the casket erupting into flames and such. I could actually see Dumbledore coming out of this alive.
Jam it back in, in the dark.