From Thursday, December 27, 2012, nationals from countries that require a tourist visa for either Thailand or Cambodia before entry (many do not), can apply for a single visa to visit both countries. These will be available at both Thai and Cambodian Embassies and consular offices.
A scheme has been in the making since 2007 for a single visa to cover the “CLMV countries” comprising Laos, Cambodia, Myanmar (Burma) and Vietnam. This has not yet been ratified, but it seems that Thailand may also become part of a future plan and the Thailand-Cambodia single visa is something of a test.
Passport holders from 35 countries are eligible for the single visa: Australia, Austria, Belgium, Bahrain, Canada, China, Denmark, Finland, France, Germany, Greece, Hong Kong, Iceland, Ireland, Israel, Italy, India, Japan, South Korea, Kuwait, Luxembourg, Netherlands, New Zealand, Norway, Oman, Portugal, Qatar, Spain, South Africa, Sweden, Switzerland, Turkey, the United Arab Emirates, United Kingdom and United States of America.
How this visa works in practice when entering or crossing the border between Thailand and Cambodia (and returning?) is unclear.
More visa information and requirements for US citizens and other nationalities are on the R-A website, including some details of China‘s new TWOV (transit without visa) for tourists from 45 countries with an onward flight ticket to and any required visa for a third country. Available at Beijing International Airport only.