well the bats in minecraft isnt a mob,they just flying objects.
tested with cages(IRON BARS) and some other experiences.
after all,the mobs does not disappear when u leave the current chunk,it just stay,but the bat goes away,so he isnt really a mob.
*cough*ejm*cough*
the bats has got changed as the update listed :-
12w38a Bats were added as object.
12w40a Bats were changed to neutral mobs, and drop XP.
as u see,it turned to NEUTRAL MOBS,so my words still right and it wasnt mob,but it is now