Select your cells and go to the number format dialog box and select Custom from the list on the left. Then in the Type text box, enter:
The entry before the first semi-colon specifies the format for positive numbers, the entry between the first and second semi-colons specifies the format for negative numbers. Change these to the correct format for your own data. The third entry specifies the format for zero.
If you format your cells this way and type 0 into the cell, the cell will CONTAIN zero, but will display whatever text you specify.
This will only work if the cell contains 0. I could not think of any way to do this for a text entry without some kind of "helper" function in another cell as jmaglasang suggests, or without some VBA code on the Worksheet_Change event.
If you would like a VBA solution, let me know. However, this means that users would have to have macros enabled for that to work.