He *knows*

[Amanda]

Hah-HAH! Yet another bit of two-by-four to add to the scaffolding I'm 
putting in under my castle-in-the-air belief that Alan Rickman's 
portrayal of Snape is accurate to *canon* and not simply an 
adaptation for film.

>From 
http://veritaserum.com/movies/actors/interviews/danielradcliffe02-14-
04.shtml

Daniel Radcliffe said:
Speaking of things to come, has JK Rowling shared any secrets about 
the conclusion of the series with the young star? "Not at all!" 
Radcliffe exclaims with finality. "She's not giving anything away. As 
far as I know, the only person on set who who knows something about 
their character's future- or past, as the case may be- is Alan 
Rickman. I think she may have told him something. I don't know what 
it is, but it's very striking!"

Hah.

~Amanda