Via CBS's Mark Knoller:
Pres Obama boarded Marine One without returning salute of US Marine at the steps. The Pres then came back out to shake the Marine's hand.— Mark Knoller (@markknoller) May 24, 2013
Pres Obama chats with and shakes hand of US Marine whose salute he didn't return on boarding Marine One today. twitter.com/markknoller/st…— Mark Knoller (@markknoller) May 24, 2013
UPDATE: More from the pool reporter:
Under gray skies and intermittent drizzle, President Obama boarded Marine One at 9:30 a.m. EDT in an open press event.
A few White House regulars were atwitter (and on Twitter) when the President walked directly up the steps of Marine One without saluting the Marine on duty. He soon came out of the helicopter, walked down the steps, shook hands with the Marine and engaged in a brief conversation.