Jeremy,
No actual geographic data is stored in an MXD file. It is generally stored in a database (Access, Oracle, Informix etc) or a "Shape File".. Or a few other possibilities... ArcGIS products (in your case, ArcMap) are used as viewers (among other things) for this geo-data.
Do you want to extract all the data for referenced in any given MXD?
Do you want to extract a particular data set referenced in a particular MXD??
If you know your data source, populating a list box straight from the raw data should be fairly easy.
If you can find out more I'll be happy to help out.
Mark.
