I once wrote a hack for this, as have others, requiring the use of three or four events...to make the prompt appear...then disappear when the Control is entered, etc...then checking, in the Form_Before Event to see if the prompt was still populating the Textbox...if so, setting the Control's Value to Null.
Then found this ultra-simply way to accomplish the same thing:
In the Control's Format Property simply enter
@;"Your Prompt Goes Here"
Prompt will disappear when Control is entered...reappear when Control is left, if no data has been entered. Also, if no data is entered...the field bound to the Control remains Null.