I hate engaging this stuff. But to add to what Josh Marshall has to say, even though Barack Obama Sr. was not a US Citizen, Obama Jr.'s mother, Ann Dunham, was a US Citizen and had lived in the US long enough to transmit citizenship to her children. You can read all about it on the State Department's web site.
Consider this another reason that Democrats need to get off their duff and pass a health care plan. The further down this birther crap gets pushed the better.