At first I thought it was an airvent. However it was glued to the inside of the bag with no opening in the mylar bag.
Then I speculated that it was a dessicator, something like those little bags of silica to trap the moisture. It wasn't one of those. It was a button, and the white center part was porous like a cigarette butt.
I was curious enough to email the customer satisfaction email on their website, but I have gotten no response. I should have phoned the 1-800 number.
Update: I did get a response. You can read it by clicking on: http://cosmologicalcabbage.blogspot.com/2009/09/thing-in-coffee-bag-explained.html
After a little bit of cogitation, my imagination ran away with me, and my thinking went like this. Suppose they were boosting the caffeine content, and the little button was like an IV drip to slowly release more caffeine into the coffee.
Then it struck me that what I had thought of was a viable invention. There have to be some applications where you would slowly release additives into the food. A good example would be vitamins. Some foods have a vitamin content that degrades with time. Suppose you had a little button like that to time-release additional vitamins into the food.
When you start seeing these things in the supermarket, remember that you read it here first.