OK I need an in-cell drop down (or even a combo box would do...) with values
populated from another workbook that resides on a public server.

Using validation only lets me have an in-cell drop down list with values
populated from a range of cells on the same sheet in the same workbook.

Basically, what I'm doing is my users will have the sheet with the in-cell
drop down list and I want to dynamically control what they see in their drop
down list. I can dynamically control a VLOOKUP using this method but not
their lookup value for the VLOOKUP.