Here's an excerpt from http://quickfacts.census.gov/qfd/meta/long_68176.htm

Definition:

The concept of race as used by the Census Bureau reflects self-identification by people according to the race or races with which they most closely identify. These categories are sociopolitical constructs and should not be interpreted as being scientific or anthropological in nature. Furthermore, the race categories include both racial and national-origin groups.

The racial classifications used by the Census Bureau adhere to the October 30,1997, Federal Register Notice entitled,"Revisions to the Standards for the Classification of Federal Data on Race and Ethnicity" issued by the Office of Management and Budget (OMB).

White. A person having origins in any of the original peoples of Europe, the Middle East, or North Africa. It includes people who indicate their race as "White" or report entries such as Irish, German, Italian, Lebanese, Near Easterner, Arab, or Polish.

Black or African American. A person having origins in any of the Black racial groups of Africa. It includes people who indicate their race as "Black, African Am., or Negro," or provide written entries such as African American, Afro American, Kenyan, Nigerian, or
Haitian.

[Italics added by me for emphasis]

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Also, here's an excerpt from http://www.migrationinformation.org/Feature/display.cfm?id=409

"Arab Americans are the immigrants (and their descendents) from the Arabic-speaking countries of the Middle East and North Africa. Under this classification, Arabic-speaking countries include the members of the Arab League and range from Morocco in the west to Iraq in the east (see sidebar). Individuals from Iran and Turkey, where the predominant languages are Farsi and Turkish, respectively, are not considered to be of Arab origin even though these countries are part of the Middle East.

According to the US Census Bureau, Arab Americans are those who responded to the 2000 census question about ancestry by listing a predominantly Arabic-speaking country or part of the world as their place of origin. The main Arab-speaking countries cited in the 2000 census included Egypt, Iraq, Jordan, Lebanon, Morocco, Palestine, and Syria...

Unlike Asian, white, or black, "Arab" is not a racial category for the Census Bureau. Rather, Arab Americans are considered white, defined by the Census Bureau as "a person having origins in any of the original peoples of Europe, the Middle East, or North Africa." This distinction dates back to court decisions from 1913 to 1917 on the "whiteness" of Syrian and Palestinian immigrants.

Arab Americans who received only the short form of the 2000 census, which is sent to all US households, could check the "white" box for race; if they self-identified as "other" and then identified themselves on the long form as a person from the Middle East or North Africa, the Census Bureau reassigned them to the "white" category. This classification system is in line with other federal guidelines on race and ethnic standards, as set out by Directive 15 by the Office of Management and Budget, and therefore is present in many administrative forms.

[Italics added by me for emphasis]