Ooh, nice theory, Roger.
The only theory I'd come up with is that Snape made an Unbreakable Vow with Dumbledore. JKR *does* like to introduce things for them to come up later... So who knows.
AnnaBtG. (who loves all the speculation, but still hopes that JKR will manage to surprise her in Book 7)
Oh, me too! It was great for me to have read HBP without any spoilers - I hadn't even known that someone was going to die. :p So I'm really hoping to be surprised.
Sara