yes they are. i have been short b on an imbalance before. i have never traded a since it trades in 10 lots but i know people who do and they are long and short it all the time.
brk.a trades in 1 share round lots, and both a and b are almost always available to short...my coach trades both brk.a and brk.b almost everyday (long and short)....the misinformation on this board is staggering at times