AFAIK, you can't do this in the cell.
You might be able to work it out with the Drawing Tools, depending on what
version of Excel you are using.
I created a Rounded Rectangle that matched the cell borders--You can force a
shape to grab cell borders if you hold Alt while you drag out the drawing.
Then, I went to the Fill Color option on the Drawing Toolbar.
If you choose more Fill Colors, you can set transparency, which would make
your text visible.
I'm using Excel 2003, and I seem to recall that the transparency choice
wasn't added until 2002. If you are using an earlier version of Excel, it may
not be there.
This seems like it would be a lot of work if you have to do it for a lot of
cells.
tj