I was reading through a post on windshield installation earlier. This started me thinking about the glass in my vette. From the inside looking out behind the mirror you can faintly see the word CORVETTE tinted into the glass. I have not seen this in other sharks although I don't think it can be seen from outside of the car.
I don't believe the glass has ever been replaced and this is definately not a decal of any sort. Could this have been some sort of dealer option available? I don't recall ever reading about this as such.

How large is it?
It's not anything original, or factory done. It is possible, that some sort of w/shield decal was on it, at one time, and the lettering kept the sun off of that part of the w/shield, leaving the unfaded area with the faint lettering. (I have a club "decal" that I use when showing my car. Its a static cling type, that goes across the top of the w/shield.)
The top of the w/shield is shaded darker, for more sun/glare protection. A decal in this area would keep the sun from fading the area with the decal on it, and it might be the reason you have a visable "Corvette".

Joel, you may have something there. These are block letters 2" high. I never noticed it until it was pointed out to me by a friend one day. (It's pretty light and as I said it is right behind the mirror. It isn't a bad thing at all,it actually looks pretty cool.