I have posted this link multiple times now but felt you may still think its interesting and somewhat relevant to your query. The link lets you put in a surname and shows you the most prevalent countries, regions and cities where it is to be found. Here is the link;
http://worldnames.publicprofiler.org/Default.aspx

The website listed these cities as the top places where the Caldwell surname are found based off of census data.
BELFAST , NORTHERN IRELAND , UNITED KINGDOM
GLASGOW , SCOTLAND , UNITED KINGDOM
LIVERPOOL , NORTH WEST , UNITED KINGDOM
EDINBURGH , SCOTLAND , UNITED KINGDOM
KILMARNOCK , SCOTLAND , UNITED KINGDOM
CHARLOTTE , NORTH CAROLINA , UNITED STATES
PAISLEY , SCOTLAND , UNITED KINGDOM

Hope this is at least moderately helpful or at the very least entertaining for you.
MEMPHIS , TENNESSEE , UNITED STATES
CHICAGO , ILLINOIS , UNITED STATES
NASHVILLE , TENNESSEE , UNITED STATES