There was a post about something similar over at Pete Freitag's website:
Calculating Distance in Miles from Latitude and Longitude
Basically, you can use Pythagorus's theory as suggest by David (the link above shows a way), as long as the Longitudinal lines are relatively similar at the northenmost and southernmost latitudes that you will be considering.
Otherwise, you need to use more complex trigonometry to account for the curvature of the earth, as shown in this page:
Distance Calculation latitude longitude global database lists