1. Set up the Customer Businessss Object, and set all String fields to Trim Right in the BO mapper
2. Set up the products  Business Object as normal (i.e. dont Right Trim any of the fields)
3. Change the field that is the source of the drop dwon (in my example I now used the prod_name field...which has data in it)
4. Pick say the first 4 records of this table and add a few blank spaces to the end of the the prod_name
5. Rebuild the project and test
6. If you select an item from the drop down , and select one of the records that you have added a few blanks, two things happen that I would not have expected
a) The drop down field itself becomes blank
b) When you leave the drop down field, an error is thrown
  Invalid Cast Exception was unhandled by code
...unable to cast object of type 'ssytem.dbnull' to type system.string