gstat distance on a non-spherical Earth?

Hello,

The interpolation routines of gstat use great circle distance in km as 
units for the distance between two  points of unprojected data. My 
question is, if the datum is not a sphere, how is distance in gstat defined
for unprojected data? For example, how does gstat calculate the "great 
cirlce distance" between two points on the WGS84 datum?

Best regards,

Dr. Martin Ivanov
Eberhard-Karls-Universit?t T?bingen
Mathematisch-Naturwissenschaftliche Fakult?t
Fachbereich Geowissenschaften
Water & Earth System Science (WESS)
Keplerstra?e 17, 72074 T?bingen, Deutschland
Tel. +4970712975023