To quote tablet_eraser: "As others have said: WARRANT officers hold a warrant from the Secretary of State for Defence here in Blighty, and from the Secretary of the Army/Navy/Air Force in the US. They are, therefore, by definition, clearly, undeniably, NOT commissioned, a commission being a document from the Head of State (HM or Bush) appointing someone as an officer. Ergo, Warrant Officers will not be saluted by British personnel of any rank, at any time, for any reason."
As has been said by other from the U.S. military, except for WO1, U.S. Warrent Officers (WO2 - WO5) ARE commissioned by a Head of State, receiving their commission from the President and having it confirmed by the Senate. Only WO1 receives a Warrant from a service secretary.
This whole conversation reeks of British snobiness. Just like calling your enlisted "other ranks" like they are insignificant.
You expect us to recognize and respect your armed forces' customs and structure. You should respect ours in return. Should an enlisted U.S. soldier not salute a British Pilot Officer because he has a funny rank? (In the U.S. soldiers opinion.) After all, both the Pilot Officer and the U.S. Warrant Officer (WO2 - WO5) are commissioned by Heads of State.