Steve, you can remove the "walls" on a load bearing wall, just not the support. Proper header size must be determined or I believe, the rafters or trusses can be redesigned to support themselves as you would if you removed a flat ceiling and made a cathedral ceiling out of it. And as BC said, you may not even have any support walls depending on the span and the material used even though the walls are at right angles to the ceiling joist. But as a contractor, I can see why your standard answer is "NO". Unless you have an structural engineer or architect do the calulations, your right, it's a risky business. He could end up with a huge skylight! BC is right, you can have walls that are at right angles to the ceiling joist that are not supporting and the example of the hall wall (supporting) and the bathroom wall directly across from it (non-supporting). That's an example, not a law. Could be the other way around. There are exceptions to that too but like BC, I'm having a lot of trouble explaining this since you can't see my hands and I can't see the expression on your face!
If I confused you then we're even, because I am too. Of course, none of that applys to mobile homes.