Hi ,
If a cell has Data Validation associated with it , it means that you are allowing the user to enter data in it , and this user entered data will be validated by the DV rules that have been created. If the DV rule is a list , then the user is allowed to either enter data which is allowed by the list , or select any item from the list.
A cell which has a formula already entered in it , can now have a DV rule associated with it , but the moment a user enters data into the cell , the formula will be overwritten with the user-entered data.
Narayan