For US developers, this would be helpful in combination with a dataset of zip codes and their lat/long coordinates (this is one I've found useful: http://federalgovernmentzipcodes.us/).

Additionally, you could adjust the function a bit and compare the distances between zip codes. Microsoft even provides a stored procedure for this functionality: http://msdn.microsoft.com/en-us/library/ms980211.aspx.

I use a modified version of this whenever I want to find the zip codes within N number of miles of a zip code. When we open a new hospital, we will often use this to determine what zip codes will be best served by the hospital.