Actually, I just found the PV Model Technical Reference, which states the following (pdf pages 13-15):
URL:
www.nrel.gov/docs/fy15osti/64102.pdf
DNI: The portion of the solar irradiance that reaches a surface normal to the sun in a direct line from the solar disk (typically assuming a measurement device with a 5? field of view), called beam normal or direct normal irradiance.
DHI: The solar irradiance on a horizontal surface from the sky excluding the solar disc.
So, looks like DNI is calculated as expected (pointing towards sun), while DHI assumes horizontal surface (which, upon reflection, makes sense from the name as well...). SAM uses the Isotropic, HDKR or Perez...I'm going with HDKR for now.
Someone please correct me if I misinterpreted the document.