Mine has this issue, but it never really bothered me until I needed to take some detailed photos for Science.
Are Apple replacing the phones for this issue, or do all phones have it?
In all fairness, if you are respectful enough to the genius and you stress how much this "issue" you are having is upsetting you, they will replace it.
Apple has always been amazing with customer service and they always will be. We pay more for our products, but (at least for me) this the reason why I continue to buy their products.
And to give you some first hand insight, my iPhone 4 is the second one I've had. This is when the proximity sensor issues were not really getting addressed by Apple and I went in for a genius appointment. I also had about 10 "test" pictures I took in different lighting with the green/blue blob to show the genius when I went in. He replaced the phone for me, no questions asked after he saw the pictures but he didn't try to replicate the proximity sensor issue.
Now, I'm not sure if he would of still replaced it if I hadn't shown him the photo's, but the point is that if you show them enough proof and treat them with respect, it goes a long way.
EDIT: Also to add, I'm still having green/blue blob problems with this new replacement. I think it might be an issue with a majority of iPhone 4's. Just my 2 cents.
Good luck.