This 'story' has only been posted about 20,000 times since 9/11.
Always check www.snopes.com (urban legend source) before posting a rumor or email, as truth, p.
http://www.snopes.com/rumors/pershing.htm
General Pershing's patriotic message was recorded at American field headquarters during the battle of Picardy and Flanders on April 4, 1918.
Thanks for posting.
jm