As I understand it, the HTTP Spec regards GET as idempotent although it is possible to write a doGet(..) method which is not. However a GET should not really change anything on the server; as it's name suggests, it should only really retrieve information. Conversely POST is not idempotent since the data it contains may be used for an irreversible transaction. Hence it's not considered Idempotent.
Hope that helps.
regards
Mo
SCJP 1.4, SCWCD (in Progress)