You should be able to continually update your file until the stated deadline on the application, whether the update is SAT, grades, activities, honors.
Except in very unusual circumstances, candidate files not completed by March 1 will not receive further consideration for appointment. This would include submission of college entrance exams.
It is my understanding that in the past, candidates for USMA and USNA (at least) could continue to update their files with test scores through the January SAT and February ACT.
When taking SAT/ACT this fall and winter it is important to have results sent directly to the academy. Request this when you register for the test. Do not wait until you get your scores.
For college entrance examination tests, the Naval Academy code for the SAT is 5809 and the ACT is 1742.
December 2008 is a good rule of thumb because if you wait for the exam dates in 2009 you run the risk of not getting the scores into your record before the Admissions Board adjourns.