Okay the title should keep those who really don't want to know away.
After the credits we go back to where Dr McTaggart is looking after the patient in a coma who was born with no conscience, basically a clean slate. Remember earlier in the film where Prof X was asking his students if it would be ethical for the mind of a family man to be placed into this body without a mind and we see Dr McTaggart and the man on a video. Anyway, the man speaks and it is Prof X's voice "Hello Moira" and she replies "Charles?", obviously hinting at an X4 film which I have heard is now supposed to be happening with his return. I heard some nonsense that the man in the coma was Prof X's brother which would be convenient for Patrick Stewart!
Only a few people stayed behind until the end and although it doesn't sound it the scene is quite integral, the cinema staff should say something when selling tickets.
If you read the comics (please I can't be the only geek when I was younger) Phoenix did destroy people but as she did it by disintegrating them their molecules could be and were resurrected. Prof X died a few times in the stories but was always brought back.
Also in the film the fact you don't see Jean Grey/Phoenix kill Cyclops means he might not be dead. Plus the fact Magneto moved the chess piece means the cure is only temporary. As seen when Dr Hank goes near Leech and his hand becomes human again, but only for a while.