How do sites like hotwire, orbitz, travelocity, etc. get access to
reservation systems, especially airlines?  They all seem to use
similiar or identical systems.  Basically, how do their systems work? 
Do they use some sort of clearinghouse or wholesaler(s)?  I need
details such as companies, resources, etc.

Many of them have paid to access Sabre, the largest service of this sort:

From sabretravelnetwork.com:

Sabre Travel Network is the leading provider of innovative travel
distribution solutions for travel agencies and suppliers. Sabre Travel
Network also manages the industry-leading Sabre® global distribution
system (GDS), the world?s largest electronic marketplace where
multiple travel providers display information about their products,
and warehouse and manage inventory.

The Sabre GDS is a primary component for travel and transportation
information for over 50,000 travel agencies, major travel suppliers,
Fortune 500 companies and travel Web sites around the globe. It serves
as a distribution source for travel content from more than 400
airlines, approximately 60,000 hotel properties, 41 car rental
companies, nine cruise lines, 36 railroads and 200 tour operators. The
Sabre system provides users with schedules, availability, pricing,
policies and rules, as well as reservation and ticketing capability
for travel suppliers. In 2003, more than 980 suppliers displayed
information about their products and services in the Sabre system that
represented more than $70 billion of products and services sold.

[http://www.sabretravelnetwork.com/about/overview.htm]

The larger travel sites have access to developers whom no doubt have
developed a way to use XML feeds direct from the tour operators. This
way they can interpret the results in any manner they wish.

Some take a nightly data dump of all of the available information, but
most link real time into the reservations systems. That's why you get
the annoying wait (with subliminal advertising) whilst they search for
your flight, hotel, car or whatever