I have only ever seen 727s, MD80s, F100s and DC-9s do powerbacks from gates. Yes, I have seen AirTran 717s powerback in ATL so I know its possible, why wouldn't it be?
Also, from what I see, the reason a 737 (or other aircraft with wing mounted engines) couldnt do a powerback may be a combination of two things. The type of thrust reverser on a 737 is different from the clamshell types on MD-80s etc, maybe it doesnt give the right kind of "push" and mostly, I think that the possibility of the engine sucking in FOD on a 737 is much breater since its SO close to the ground, everything stirred up from the thrust reverser is going to go right into the engine!
Delta- replace your 737-200s on the shuttle runs with 717s!! Also, how about some 717s for SkyWest, those damn little CRJs are so full out of Salt Lake City to points north that you have to sometimes wait all day for a seat to a market like Boise, Billings or Calgary- wait, sometimes I have to wait 2 days for a seat to Calgary!