Update: 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 is the protocol from the Queen of England's website -- which applies even to her own subjects:
"The Queen meets thousands of people each year in the UK and overseas. Before meeting Her Majesty, many people ask how they should behave. The simple answer is that there are no obligatory codes of behaviour - just courtesy.
However, many people wish to observe the traditional forms of greeting.
For men this is a neck bow (from the head only) whilst women do a small curtsy. Other people prefer simply to shake hands in the usual way.
On presentation to The Queen, the correct formal address is 'Your Majesty' and subsequently 'Ma'am'. "
Please tell me he didn’t do the “as-salaam alaykum” thing.
http://www.youtube.com/watch?v=BYLuLEfVNow
OBAMA BOWED BECAUSE Her Majesty presented Mr. Obama with his Kenyan birth certificate from Mombassa hospital that shows all the facts of his birth and, most importantly, that Barack Hussein Obama Seotero is not a native born American citizen, but a Subject of the Crown by way of his Kenyan father's British Colonial citizenship.
Her Majesty then gave the usurper his orders and he will comply or be exposed as the fraud he certainly is.
He's not a "natural born citizen",
plus this
equals this