Okay so first of all, here's the wikipedia entry for the G-spot with a nice drawing of the female anatomy:

en.wikipedia.org/wiki/G-spot

Do you mean the clitoral hood? This would be the little flap of skin that often, but not always, covers the clitoris, often when not fully erect. The size and shape of the clitoral hood can vary greatly from none at all to completely covering the clitoris, even when erect.:

en.wikipedia.org/wiki/Clit..._formation

Or if you really do mean the G-spot, the exact location can vary from woman to woman, and it's internal. Is this Cheerio-sized opening you're talking about internal?

Just trying to narrow down your question a bit!

Are you sure you're describing the g-spot? What you're describing sounds an awful lot like a cervix to me. The "cheerio" is a classic description of the os (Latin for "eye"), which is the opening that leads up into the uterus. And, no, it's not ordinarily more sensitive as far as I know -- however, many women dislike the feeling of rough handling or bumping of the cervix.

The g-spot is on the front wall of the vagina, and feels a little bit like the side of a walnut buried under the tissue of the vaginal wall. To find it, imagine the woman lying on her back. Extend your index finger, palm up, and reach into her vagina with the finger as far as you can. If you crook the finger in a "come here" gesture, you'll be on or near her g-spot. (For contrast, the cervix would be behind your index fingernail or at your fingertip when the finger was extended straight out.)

There's a picture at i36.photobucket.com/albums/e...gspot.jpg that shows the g-spot pretty well. Unfortunately, it's less clear about the location of the cervix and os. There are lots of pictures of the os online (do an image search on "cervix" and "os"), but they don't do a very good job of showing its location.