found on Snopes:
Although the Walter Reed Army Medical Center is not accepting holiday cards for recovering soldiers, the American Red Cross has said that small gifts may be sent to patients in that facility through them:
The Walter Reed Army Medical Center is ONLY accepting phone cards, CDs, individual small packets of candy, things like that; which they will distribute among the soldiers (as they have a lot of in and out patients). They are not accepting Holiday Cards, as they don't have the human resources to distribute them.
You may send your packets to:
American Red Cross
Walter Reed Army Medical Center
6900 Georgia Avenue NW
Washington, DC 20307-5000