Truth about Snape?

[chrusotoxos]

No: HPFGUIDX 125297


Hi everyone!
I haven't been in there for quite some time (exams coming, groan) but at a first glance it 
seems to me that nobody is discussing this:

"With something huge revealed about Lily Potter, the truth about why Dumbledore trusts 
Snape, and a little romance for Harry, this promises to be one of his best years at 
Hogwarts yet."
(source: Mugglenet)

Question is, why????
I mean, who cares about Lily (ok, nice Lily, and I'll love to know something more about her) 
but we'll know in Book 6 the truth about Snape??! I was prepared to drool on it till book 7, 
which will probably be published in 2011...
So, why Dumbledore trusts him? What did he do? And why do we know it in Book 6? Harry 
will finally trust him? Or Snape will die in Book 6?

Please answer this!