One of the annoying limitations of Excel is that Data Validation does not work from one sheet to another. If you have a list of items that you want to use as a pick list for a specific cell, the cell and the list need to be in the same worksheet.
But apparently not. If the list is a named range, and you refer to that named range in the Data Validation, then it can sit in a different worksheet.
Thanks to Lee Smyth for this one.