Hi-
Once you have your connection to the SharePoint list set up, all the data is stored as a secondary data source in your form. Using rules and filters, the selection in the dropdown can be used to retrieve the matching record from the secondary data source. If you look up cascading dropdowns you might find quite a few tutorials on the process. I found this video which might help you:
http://blogs.msdn.com/b/infopath/archive/2010/03/04/use-rules-and-filters-to-create-cascading-dropdowns-in-forms.aspx
Good luck.