This afternoon, a twitter account was created for the Mark of the Spider-Man. Soon after, it tweeted a message that included coordinates.
Since then, four more tweets have gone out, each with two locations in one state. So far, Peter has lost items in New York City, Atlanta, two locations in Colorado (Denver and Glendale), Phoenix and Seattle.
A Twitter user in Atlanta found some of the lost items. Below you can see that there was a backpack filled with shoes, a sketchbook and other various items (Follow @dag_kurt for more pictures)
Be sure to follow the official Mark of the Spider-Man twitter account for more clues.