Good question! 

I read this recently but interpreted it differently.  Specifically, I took the 'snipe attack' comment as referring to an attack that takes the target completely unawares (regardless of whether the assassin is using a melee or missile weapon).  That is, the term 'snipe' is metaphorical rather than literal.  According to this interpretation, someone in combat is assumed to be vigilant for danger and so immune to an assassination attempt of any sort. 
 

Yes, concur with that.  Re-reading the section, I see the missile weapon link.  (The 'snipe' comment was in a separate sentence, and I'd somehow disassociated it from what came immediately before; but examining again, the context with missile weapons makes much more sense).
 

Jeff will correct me if necessary, but here's my understanding: For attacks with class 1 or 2 melee weapons, no surprize roll is needed; the target only need be "unaware" of the attack. (The referee could rule that a target in the midst of melee with the assassin's companions might be unaware of the assassin hiding and moving silently behind him.) If the attack hits (with a +4 bonus), check the Assassination table for chance of instant death. Even if instant death does not result, normal backstab damage applies, with a multiple based on the assassin's level.

​Assassins also can assassinate with a missile weapon, but only at short range, and only against a target who is not engaged in combat and does not expect danger. In other respects, the attack is resolved as above.

DMP is spot-on, as usual. I do not require a surprize check if the target is unaware of the assassin's presence, and the assassin has successfully put himself in position through move silently and/or hiding.