Probably Obama’s ancestor is Chester W. Payne:
Payne, Chester L. Inducted 31 January 1945, Army 37760745 Registered, order # 12622-A Wichita, Sedgwick Co. (Board # 3)
Sedgwick County is where Obama’s grandfather lived, according to Ancestry.com.
First Draft of Ancestry of Obama:
http://www.wargs.com/political/obama.html
His grandfather:
Stanley Armour Dunham, salesman, born Kansas, 23 March 1918, d. Honolulu, Hawaii, 8 Feb. 1992 [SSDI 514-03-4824], bur. Punchbowl National Cemetery, Honolulu, Hawaii
marr. 5 May 1940
Here is the WWI Draft Registration of Obama’s grandfather:
http://search.ancestry.com/cgi-bin/sse.dll?db=wwiienlist&h=4719327&ti=0&indiv=try
BTW, my husband is related to Obama, through the Allred line from Overton County TN.
I'm not quite sure why everyone assumes the "uncle" is Payne. Stanley Dunham had a brother, Ralph E., according to the 1930 Census (on the site you linked) who, according to this World War II Kansas Veterans Index, was drafted and inducted into the Army on May 28, 1942. Stanley (listed right after Ralph) enlisted and was inducted on June 18, 1942.
Think it might be Charles E. Payne, who was inducted in 1942 from Butler County, KS, the same county which listed him in the 1930 census as a 5 year old child of Rolla & Leona Payne and brother of Madelyn, Obama’s grandmother.
Who knows, maybe Charles came home and locked himself away for a few days with a few beers and a carton of cigarettes and he has been turned into the poster child for WWII PTSD.