From the pool report:
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.
It’s possible Obama didn’t at first recognize the Marine without his umbrella. Or maybe at a future press conference we’ll learn more about the reason for the delay:
obama wasn't aware he didn't salute until he saw it on the news – jay carney
— Cthulu 2016 (@Illuminatas) May 24, 2013
Video via The Blaze:
At about the 12-second mark you might see a late and half-hearted salute for which Obama returned to make amends, or perhaps the president was saluting somebody else in the helicopter when he realized he failed to return the salute of the Marine standing at attention. Either way, a handshake was due.