I have been running with Nike+ since November 2006. Nike has just implemented the milestone functionality of the website. For running specified distances you will receive a certificate, you can either print this certificate out or share the link with friends. I have received 3 milestones, 100, 500, & 1000 miles. I am just over 1000 miles on Nike+, so I am unsure of all of the award amounts.
Here is what my 1000 mile certificate looks like:
Go forth and collect your milestones!