Since I've personally seen Corian countertops that were 20 years old that were indistinguishable from new (they had just been resurfaced), I'd be very curious as to the source of your yellowing.

Have you tried using the Scotchbrite pads recommended for scratches and actually doing a bit of sanding/resurfacing in the areas? You mention that bleach substances just dim the stains. Do the stains appear to be in the surface of the Corian, or do they appear to be coming up from beneath? What does the underside of the countertop look like? Is it stained too? Are the yellow spots over where 2 cabinets come together? Maybe in a line that follows the edge of the cabinet?

Do you have any picturea? If you can think of any more clues, let us know.

If bleach seems to be removing the stains, I would put a bleach-soaked paper towel or cloth on the stain and cover it with plastic wrap (Saran or similar) for a few hours or longer, even a day or so. I did that once to deal with a stain on a slate coffee table top, and it took the stain out completely.

My mom has corian that was installed 5-6 years ago. She too has some yellowing around her sink. I think it might be staining from something in her water. She has a cleaning service weekly and keeps her kitchen clean so I don't think it is mold or gunk. She has a beige-ish corian with smaller white speckles and some larger darker tan speckles.

Full strength bleach is NOT a good idea on acrylic composite. Dilute the bleach with water and soak paper towel and then leave on for just a few minutes, then scrub and rinse clean.

Bleach can actually cause acrylic/plastic to yellow.

Here's a tip I found during a Google search:
Bleach Stain Removal from Countertop:
(from www.cleaning.lifetips.com)
Kitchen cleaning is more than washing dishes. One also must also consider cleaning kitchen cabinets and counters, which can become stained if not properly maintained. To remove a yellow bleach stain from your countertop, \ try using a paste made from baking soda and water. Let it sit for half an hour, then scrub the area with a damp nylon sponge. Bleach causes a permanant reaction however, so your efforts may be futile.

Kompy

Yes, that's something I forgot. :~) Areas with high iron content to their water shouldn't use any bleach at all on their countertops or sinks---of any material. Bleach actually accelerates the rust formation of the iron content in the water. Using an oxalic acid based product like Barkeeper's Friend is what's recommended to deal with stains in high iron areas. It may also help remove the yellowish rusty stains caused by chlorine setting the iron stain into the surface of the material. If Barkeeper's Friend doesn't work, then resurfacing the material may be your only choice if the stains are just in the top portion of the countertop. If the acrylic has been damaged by the bleach deeper, then there really isn't much you can do.

I appreciate all the comments. I am off to buy some of the mentioned products. I have tried the bleach with paper towels with little result. There is no yellowing under the sink or cabinets. The main yellowing is around the side of the sink with the disposal although the other side also has yellowing spots and at the inside of the countertop next to the backsplash. The grout has also turned yellow. It cleans up less well than the Corian. I did try putting amonia on the areas; the countertop did seem to look less yellow.

does anyone know how to tell if your water has a high iron content?

I tried the Bar Tenders Friend and that has helped the most. I called the place where I bought the Corian and they are sending a kit; they will come out if that does not work. I really think the Clorox Cleanup I used caused a lot of the problem and yes our water does have a lot of iron and chlorine in it. The splotches in the white sink are invisible now until they get wet. Also, I cleaned with a white cloth using the Bar Tenders Friend and my cloth has yellow spots now. I am also wondering if dirt from a plant I let sit in the sink may have also added to the problem.

bleach is what caused the yellowing of my sink and I believe I will actually have to use a fine sand paper inasmuch as the yellow area is also etched.