ok, Here's what I "think" I know. As stated in a post above, the whole plane is pressurized - pets in cargo- I think it's about the same as being at 8000ft or so I've been told.
I just flew home last week and happened to have my mosquito on. I've heard horror stories about computers getting hosed when they get turned on at altitude - in fact, had a friend tell me it happened to him. I wondered about this and wanted to see just what happened to them and had a quarter in my pocket so I could pull the battery if needed.
I put the mosquito in dive mode for regular air as we were starting the approach. I thought that this would make the computer think we're at sea level when were're theoritically at about 8000ft. As we drop, the computer should start thinking it's diving and regester X fsw. Durring the decent, the computer never regestered that we were going down and, in fact, it turned itself off from inactivity and went back into wristwatch mode just before we landed.
Now this does nothing to state how an anolog guage would act, but I think I've just de-bunked that computer and flying thing.