My Co-Worker Bought Me A Gift

Here is the daily dilemma for Tuesday, December 20th:

"At the office, we all basically, agreed that we weren't going to exchange gifts. We brought the situation up, but no one really said anything. Yesterday, I came in and one of my co-workers brought in this beautifully wrapped gift for me and I didn't get him anything. I don't know what to do now? Should I run out and get something or is that a little obvious?"

Has this happened to you? Should you or this person feel obligated to buy something for that person now?