Daniel Sheres at the D.C. Office of Planning has compiled all the equations for calculating ACS statistics (margins of error, statistical significance, etc.) into the following spreadsheet:
Spreadsheet to Calculate ACS Margins of Error and Statistical Significance for Sums, Proportions and Ratios3 Mar
This calculator, updated 2/23/2009, from the New York Data Center allows users to produce the margins of error for the ACS when combining counts or proportions for a number of geographic areas. It also allows users to test significant differences between two estimates.