Liz, I think nowdays most airlines that does accept pets to travel with them actually have a preassure and temperature controlled holding for them even if not as carryon. At least most of the airlines around here does, However I would always prefere to bring a critter as a carryon, but the sizelimitations here makes it more or less impossible most of the time since a normal size cat carrier is too big to be taken as hand luggage here.
Anyways just wanted to drop my 2c about the preassure and temperature for transportation. I dont know as I said any company that transports animals in an uncontrolled compartment. (still beleive a transportation in an aircraft is a ton of stress on any animal since it is one that is really hard to train for). As most bigger airplanes cargo hold is climatised and pressurized exactly the same as the passengercompratment above.
The difference between an airline suitable to transport pets or not is their routine when it comes to loading unloading. Meaning they minimize and monitor time spent waiting on the tarmac both ways, since this is where it can be hot or cold depending on where your going too and from. But on the airplane itself, temperature is not an issue, and well airpreassure isnt really an issue either on ground or in the air.
So conclusion, I think that the preassure and temperature concernes onboard a passenger airplanes cargo hold, is either a myth or old concerns that has continued circulating onto modern airplanes as well (or quite posibly I am actually wrong, I bet there is someone here who works on airplanes that might be able to shed some light on the situation)