>> Yes.

> Well, that sounds definitive.  I was hoping for a reply that would
> enlighten me a little more, however.

Well, I didn't want to waste any words.  ;-)

Seriously, this has been discussed on the list many times before.
Air pressure is inverse to altitude, so lower at higher
altitudes.  That effectively lowers the compression ratio of
the engine.  Hence, lower octane requirements.

The Wikipedia article has some good info:
http://en.wikipedia.org/wiki/Octane_rating

Of course, the best authority is:
http://www.cartalk.com/content/columns/Archive/1997/June/09.html

I would like to know how the lower octane at altitude is specified
and by whom--EPA, DOT?  I haven't stumbled on any detailed info about
that.

A good general reference on gaseoline is:
http://www.faqs.org/faqs/autos/gasoline-faq/
Note that he can be terse, too.  His answer to why we do not
have hydrogen powered cars is "The Hindenburg."

Regarding alcoholic gas, here in the Colorado front range counties
gasoline is "oxygenated" from November 1st to January 31st.  The
pumps used to have stickers appear and disappear.  Now they just
leave stickers on all year that state "may have...."  In fact, I
think that all pumps all over the state have the stickers.

Richard