New in SolidWorks 2013 is the option to use values stored in a component or sub-assemblies configuration properties as balloon text.
A component or sub-assemblies configuration properties has an option for ‘Part Number Displayed When Used in a Bill of Materials’. This is accessed by right clicking the configuration name and modifying the Bill of Material options section. The default value is the document name but other options include the configuration name or a user defined value.
This configuration specific property can now be used as the value for balloon text in a drawing. To use this value simply set the balloon to use the ‘Part Number (BOM)’ as the balloon text value.
If the values do not seem to match what is expected be sure to review the configuration properties as noted above.