US President Barack Obama, center, back to camera, greets King Abdullah of Saudi Arabia, center, before the official G20 leaders group photo with Britain's Queen Elizabeth II at London's Buckingham Palace, Wednesday, April 1, 2009.
American Thinker adds this:
See Miss Manners on the protocol. Americans do not bow to foreign monarchs because that act signified the monarch’s power over his subjects.
Here’s the video. Obama’s bow is at :53 seconds in: