This is a feature I have been requesting for the past two years. It looked as if iTunes 7 might finally do it, with displaying the explicit tag from purchased songs (incorrectly, to nonexplicit content, I might add), but no.
This seems like a no-brainer Apple, especially if you can display it for purchased music, there should be a way to turn it off/on on a song-by-song basis... that would also solve all of the problems and requests.
I suggest a good place for the option might be on the option tab of the get info window, under "my rating"... makes sense to me. oh well, we'll just have to wait for the next version so we can be disappointed again.
P.S. Typing "Explicit" in the comments field is a terrible solution, as there are already too many uses for the comment field that should be turned into true meta data, such as featured artists, and true comments such as thoughts or reviews of the song.