RE: Harry Potter Fans... Warning - Spoilers! If you h...
I agree with you and have a pet theory: I think that Snape took an unbreakable vow to protect Harry and that's why Dumbledore trusts him. Otherwise, the way the book is written it makes it seems as if Dumbledore, this incredibly smart and powerful wizard, was just a total dork when it came to trusting Snape. It doesn't fit the way the character has been written throughout all the books. Plus, she has always made a huge deal about Dumbledore trusting Snape in every book:Snape's supposedly saying that he regretted the death of James and Lilly--it's a really weak plot device to prop up something that she made such a huge stink about. So either she has something else up her sleeve, along the lines of an unbreakable vow, or she has gotten sloppy and lazy because she's too rich!
I also think that Snape and Dumbledore had an agreement that Snape would sacrifice Dumbledore if it came down to Snape having to reveal that he is on Dumbledore's side. Either it was a long-standing agreement, or something that Dumbledore had decided on after his hand injury--maybe it would kill him pretty soon anyhow. In either case, the reason that Dumbledore was pleading to Snape at the end was because he *wanted* Snape to kill him. There's a slight possibility that JK Rowling is even pulling an Obi Wan Knobi-type situation: Dumbledore will somehow come back stronger than ever when he's dead...but I doubt it--Harry has to defeat Voldemort on his own. Dumbledore can't always be there to save the day (which, by the way, is why my SO guessed that in this book Dumbledore would die--I knew the ending of the book before I read it).
Next book: Harry will of course track down Snape out of revenge and hatred, but Snape will somehow give his life to save Harry in the end, thereby redeeming himself and Dumbledore's trust in him. Harry is also going to be forced into the position of having to trust Malfoy somehow, who I think will turn out not to be evil in the end, though that could go either way.
There...basic plot outline done. Now if I could only get a couple of million dollars for it!
Love these books--so sorry to see them ending! I've read them from the beginning.