From a technical view, "All people are created equal" means that all people are equal, while "All people are created equally" would imply that they are created in the same way and does not directly state that they are equal. This is, however, the literal meaning of the phrase.
It is often used to mean "We should treat people as equals" or "All people have the same rights" and in this way both can be taken to mean exactly the same thing. As far as I can tell, this stems from the quote "All men are created equal" by Thomas Jefferson during the american revolutionary period. This line is in the declaration of independance, and has come under scrutiny for its use of the word "men":
The concept that all men are created equal was a key to European Enlightenment philosophy. But the interpretation of "all men" has hovered over the Declaration of Independence since its creation. Although most people have interpreted "all men" to mean humanity, others have argued that Jefferson and the other authors of the Declaration meant to exclude women and children. Within the context of the times it is clear that "all men" was a euphemism for "humanity," and thus those people, such as Elizabeth Cady Stanton, Abraham Lincoln, and Martin Luther King, who used the Declaration of Independence to demand equality for African Americans and women seized the historical as well as the moral high ground.