AADFW: Most of the regular posters on here know I would be the last to defend PHX. However, it looks like PHX's point was not whether some cargo holds are lethal environments for live animals, but rather your contention that cargo holds are not pressurized. Are there, really, any "mainline"-type aircraft that do NOT have pressurized cargo holds? Possibly no pressurization in the cargo hold of some commuter aircraft may be the case.
Because of the unpredictability of some airline staff I would send my pet on Pet Airways (
http://petairways.com/ ) or, if traveling with my pet, use Amtrak.