Found this on Usenet:
On the Drawing toolbar, click Insert WordArt.
Select a design and click OK.
Enter your text and click OK.
Resize the object as desired.
On the WordArt toolbar, click Format WordArt.
Set the Fill Color to No Fill. Set the Line Color to a light gray and click
OK.
Now you can enter data into cells and have a watermark-like object appear
underneath your data.