Start off by opening a new document in Adobe Illustrator(I'm using Illustrator CS2).

Type Out The Text

Grab the Text tool and type out your text. It's better to use a font with wider letters – the font I used was Arial Black.
Go ahead and give the text a stroke color.

Duplicate The Text

Select your text.

Go to Edit->Copy.

Then go to Edit->Paste in Back

Hit ctrl+2 to lock the copy of the text. You shouldn't see any kind of change.

Draw Out The Highlight

How you draw the highlight is up to you. I took the rectangle tool and drew a rectangle that covered the top half of my text. You can use the circle tool or the pen to draw out the highlight area.
Make the shape white with no stroke.

The Intersect Shape Area Tool

Now to get the highlight on the text only.

Go to Window->Pathfinder to open up the Pathfinder pallet.

Select the highlight area and the text

Next click Intersect shape areas in the Pathfinder pallet. It's the third from left under Shape Modes:

After you click Intersect shape areas click Expand (it's in the Pathfinder pallet)

Finally, go take the Opacity: down to 20 or 30 percent. And hit ctrl+alt+2 to unlock your text layer.

This same process will work with shapes, not just text. Try it out.