maybe i'm just less risk averse than you guys, but I've always just packed the guit well in the case and stuck a fragile sticker on it and sent it thru with normal baggage. I've done this with my electrics (Tele, EBMM, Washburn SE29), acoustic (Maton Custom) and bass (Yamaha 5 string). The Yamaha was even in a soft case and we went via China Air!!
I've never had a problem, and have travelled with my guits regularly. As long as they are in a good flight case and are well packed inside (ie with lots of padding so that they don't move around, but still have enough bubble wrap or rubber foam to cushion the blows), I think it's generally ok.
Mind you, having said that, there have been times when we've travelled and equipment has arrived damaged. Our EQ and vocal processor were damaged in its flight case between here and Shanghai once, but I think these flight cases don't have much "give" when they are dropped.
I actually DO loosen the tension on my strings before checking them in. Dunno... just in case. It's just one less thing to go wrong.