"I get the impression that no caulk at all is really required because a properly flashed window or door should not leak inside. Is this true?"

No, the flashing is to divert water that may get through, the caulk is to try and prevent it from entering in the first place.

What about the moisture in the house thats trying to get out?

If your caulking around windows is bad, blown rain gets into the wall, interior paint might peel.. insulation becomes damp and growing mold inside the walls isnt a very healthy thing.

I just looked at a windows installation instructions and they require expanding foam and caulk to meet code here.

~What about the moisture in the house thats trying to get out?
Depends on where youre at. Moisture might be trying to get in when you live in a hot humid area. Like here in florida. A/C already works hard trying to control the moisture, why add to it by letting more in.

Thank you. This discussion is helpful (and I think what I thought intuitively).

http://www.stocorp.com/pr.nsf/5d5efb09f77373618525666a004bca57/d4f316cb97c674368525692d006d00ea?OpenDocument