Someone who knows why DD trusted Snape?

[cara_vzq]

No: HPFGUIDX 133900

Hello all, de-lurking as I occasionally do here to say, there is a
part in the book that threw up a questions for me, over why Dumbledore
trusted Snape. When talking to Dumbledore about Snape Harry says:

`So, Sir,' said Harry, in what he hoped was a polite, calm
voice, `you
definitely still trust--?'
`I have been tolerant enough to answer that question already'
said
Dumbledore, but he did not sound very tolerant anymore. `My
answer has
not changed.' 
`I should think not,' said a snide voice; Phineas Nigellus was
evidently only pretending to be asleep. Dumbledore ignored him.  
P336, UK Edition,

Could this mean that Phineas knows the reason that Dumbledore trusts
Snape? Phineas, who has before moved between his portrait at Hogwarts
to Grimmaud Place, which Harry now owns, and will possibly be visiting
in Book 7
.
There might be potential for an explanation through Phineas.
I personally am undecided as to Snapes allegiance, there are
compelling arguments on each side, though I must admit that I'm 
hoping for a good Snape

Sorry if this has been picked up on before, there are so many posts I
can hardly keep up!

"Cara"