|Subject: Re: U.S. Transportation|
Because of the distances between cities (especially in the western US), many visitors do rent cars. This also provides you with flexibility to go off the beaten path to see places where the buses will not take you.
However, you can definitely travel between all the major cities via bus as well. Greyhound has a good site that answers a lot of your questions.
Route map - http://www.greyhound.com/maps/ Baggage information - http://www.greyhound.com/faq/baggage.shtml (note that you are limited to two carry on bags and two checked bags with the checked bags weighing no more than 50 pounds each).
In Southern California and Arizona, Backpacker Bus is an alternative to Greyhound - http://www.backpackerbus.com
The following journal might also give you an idea of what to expect on a long bus trip. It was written by a woman who took Greyhound from Denver to California - http://www.hostels.com/en/tt/walzbus.html
Cheers, Mark in Los Angeles