Do I need a passport to go to Puerto Rico 2011?

I won some tickets over the radio and now im flying to San Juan Puerto Rico April 1 2011 from Washington DC, I really want to know if I need a passport. I keep hearing I dont but some people tell me I DO. Please someone that know the current rules of this let me know. I know PR is part of the united states but im really not sure and I only have 11 days till the flight please let me know if I need a passport to go to puerto rico and to come back from PR. PLEASE ANSWER THANK YOU (: