I can't believe this didn't register with me before. In looking at the "avada kedavra" photo from the yet-to-be-released HBP film, he's actually casting the spell with his left hand.
We saw him do this once before when he slammed the shutters closed in the DADA classroom in PoA, but I remember reading somewhere that they filmed it that way so that Rickman wouldn't have to reach across himself in the shot.
Back in CoS, he dueled Lockhart with his right hand.

In GoF, he clearly used his right hand when opening the trunk containing the real Mad-Eye Moody, and when poking poor David Tennant in the face. :cP
In OotP, both the flashback Snape and present-day Snape used their right hand.


In another scene from HBP, where he's examining the necklace, he's back to using his right hand.

So, is Snape ambidextrous or what? I can't see any technical reason for Rickman to be using his left hand in that shot. So what's that all about, anyway?
Bored Inquiring minds want to know.