canonlaw: For Belgian based clients UA offers their USA airpass on
http://www.ual.be/logic.asp?ID=0202#Airpass
USA Airpass
With hubs in Chicago, Washington, D.C., Denver, Los Angeles and San Francisco, United makes it possible to fly to over 250 destinations within the U.S.A. at competitive Airpass rates. This is the easiest way to see the U.S.A. Online fares apply to transatlantic flights on United, Lufthansa, SAS or Sabena. Worldwide fares apply to transatlantic flights on any other scheduled carrier. Travel is valid within the Continental U.S., Canada, Alaska, Mexico and San Juan. Extensions may be purchased for trips to Hawaii for US$110 per segment during the high season and US$95 per segment during the low season. Fares are listed in US$. Fares are subject to change without notice. Please check with United Reservations for the most up-to-date fares.
There you find also the pricelist: low season, high season, different fares dependiding on transatlatic carrier.
Otherwise the same ruels as I mentioned in a poster above apply (30 days rule etc).
I usually buy 8 segments, low season (the seasonalitiy is defined by the first segment's date), and than a segment (which is defined as ONE FLIGHT NUMBER, even with stopovers it is just one segment) costs me an average of $79 (even for a transcontinental flight!, a max of two transcontinent flights is allowed).