This came up in a recent conversation. I've tried to include articles from both sides of the argument.
Myth Making and Spitting Images from Vietnam
Vietnam déjà vu (Revisionists deny spitting on troops)
The Spitting Image: Myth, Memory, and the Legacy of Vietnam
Spitting on Soldiers
The Myth of the Spat-Upon Veteran By Gabrielle Bernard, Winsted
(Article is no longer available, but google has it cached.)
Resolving The Spitting Debate