Below is a sample letter that would be appropriate if deferred. Caitlin has a significant new honor to report to her first choice college, so she certainly should make the school aware of the update to her application. Note that her letter is polite and concise. She doesn't express her frustration or anger; she doesn't try to convince the school that they've made a mistake; instead, she reaffirms her interest in the school, presents the new information, and thanks the admissions officer.
Dear Mr. Carlos,
I am writing to inform you of an addition to my University of Georgia application. Although my admission for Early Action has been deferred, I am still very interested in UGA and would very much like to be admitted, and therefore I wish to keep you up to date on my activities and achievements.
Earlier this month I participated in the 2009 Siemens Competition in Math, Science and Technology in New York City. My high school team was awarded a $10,000 scholarship for our research on graph theory. The judges consisted of a panel of scientists and mathematicians led by former astronaut Dr. Thomas Jones; the awards were presented at a ceremony on Dec. 7. Over two thousand students entered this competition, and I was extremely honored to be recognized alongside the other winners. More information on this competition can be found through the Siemens Foundation web site: http://www.siemens-foundation.org/en/.
Thank you for your continued consideration of my application.