President William McKinley is US president in 1900 and is enumerated while living in the White House.
Of course the census taker did not refer to it as the "White House," it is referred to as the "President's Mansion" in a notation made on the far right hand side of the enumeration. The 1900 Census is searchable on Ancestry.com.
The entry for McKinley is difficult to read, given that a statistician from the Bureau of the Census has made a notation right above his last name. This is compounded by the fact that the enumerator spelled the president's last name incorrectly (it's pretty clear that the last two letters of the name are "ly" as written in the census).
This is the screen that shows how McKinley's name was indexed---pretty reasonable for him considering what it looked like. Users can enter corrections or comments--which is what I intend to do as soon as I post this blog entry. His wife was Ida Sexton, although it is not the easiest thing to read either. If you find someone whose index entry is wrong (or that needs a comment because the original is slightly off) consider entering that comment when you find the name in a census entry at Ancestry.com. Those names eventually are searchable after the corrections and comments have been uploaded.