
Jonathan, interesting questions.  Some wonder whether or not the moon could
have ever supported an atmosphere.  I'd be interested in knowing what
our geology/environmental sciences friends think.

As for human tolerances, the best example of human endurance in terms
of altitude (i.e. low atmospheric pressure and lower oxygen partial pressure)
is in my opinion to the scaling of Mt. Everest without oxygen assistance.
This was accomplished by a team of mountaineers who trained at high
altitudes for quite awhile (I think a few months) and then were flown by
helicopter from that training altitude to the equivalent altitude on
Mount Everest, where they began the ascent of our planet's highest peak
without oxygen tanks.  This is quite a feat of physiological endurance, because
if you or I tried to go to 20,000 feet and exert ourselves, we would probably
pass out, get altitude sick, and could even die from cerebral edema. So
this is the limit of low pressure.  High pressure situations would be
limited by the duration of time which it takes to slowly acclimate to a higher
pressure.  Skin divers would know alot about high pressure situations and
could tell you about how they safely make deep dives without getting the
bends.  Some military experiments have put people under several atmospheres of
pressure (not sure what the high limit was because the papers aren't in
front of me).  Usually at a certain point, the nitrogen in the air becomes
toxic to the body and you start acting idiotic.  Divers call this nitrogen
narcosis.  Those afflicted can do very dangerous and irrational things, like
taking off a diving mask and oxygen tank in order to talk to fish at 100 feet
under water.  (Hope any diving folk can elaborate on this matter, as I
am not a diving expert).

Mars cannot support human life without pressurization because the atmosphere
is too thin (1/100 th  our Earth's atmospheric density).  In addition,
the Mars atmosphere is mostly carbon dioxide.  Basically, you would need a 
pressure suit there, or you'd die from the low pressure.  Interesting huh?