As I thread, that's an issue for a different thread, which actually has been settled because Rowling has acknowledged that he is dead. Some of the prose concerning his death seems to include extra details that make you wonder whether or not there was some other plan afoot to fake Dumbledore's death. When I read it the first time, I thought about this: "Severus, I have a great plan. First, you kill me."
He was afraid of death, but that's not the same as inviting death. Another theory that I've heard is that he was already "dead" in some fashion during the last book. And one wonders if he could somehow pull a Phoenix and rise. Renewal is a continual theme even if dead is dead. You can't apparate into or out of Hogwarts, but you can if you hold the tailfeather of a phoenix. So no rule seems to be universal.
But, again, I'll take Rowling at her word. On the other hand, if she was being cryptic and parsing her words and Dumbledore is somehow alive, stick him into my predictions where he is.
Being dead doesn't keep him from communicating with Harry through his portrait or through an as yet undiscovered horcrux or two-way mirror.
That's something that I've wondered about quite a bit. Not only can't you apparate in and out, you can't do it "on campus." Remember the apparation classes in HBP: Dumbledore had to remove the charm in the Great Hall during the class.
However, also note that the House Elves were able to apparate within Hogwarts.
Maybe it's because when a wizard aparates, he's casting a spell, which a Phoenix and House Elves are magical creatures, and apparation is part of what they are? Just a guess...
Man, I'd LOVE to have some HP conversations, especially today! I've been fighting IP routing and VPN fires last night between 6:00pm and midnight, and all day today!