A small clarification to the last...
SFI145's formula is completely correct, but for the benefit of those at the back of the maths class who may not have been keeping up, I'd add a small word about units, especially for those of you typing the formula into your favourite spreadsheet.
Your favourite spreadsheet probably does sines and cosines of angles in radians, and the formula shown works great if your Lat and Long pairs are in radians, at which point it also gives you the answer in Great Circle radians.
For those of you who started with Lat/Long in degrees, and wanted the answer in nautical miles - first convert your degrees into radians by multiplying by (pi/180), and convert the radian answer back into degrees by using the opposite (180/pi). Finally, to get from great circle degrees to nautical miles, multiply by 60.