As you might, and might not have seen on youtube theese last few days, I've been playing around a little with the piston mod.
So yesterday, after work, in between yelling at my friends being assviolators on me and MedicAtk's server I had a little time to do some more playing and testing with the pistons.
I tested if you could kill a player with them, so I bravely put myself between 8 pistons and activated the! Nothing happened to me though.
And yesterday a guy called muiic on youtube tested if he could squeeze some zombies to death with pistons... he could not.
I think we can safely assume that it is not possible to kill mobs or players with the pistons itself.