This is an excellent example of a really, really funny prank that also happens to have a positive social message on a number of fronts. Here's the scoop, via Michael Arrington at TechCrunch, who explains how someone could just hack into a presidential candidate's MySpace page:
Someone on Presidential hopeful John McCain's staff is going to be in trouble today. They used a well known template to create his Myspace page. The template was designed by Newsvine Founder and CEO Mike Davidson (original template is here). Davidson gave the template code away to anyone who wanted to use it, but asked that he be given credit when it was used, and told users to host their own image files.
McCain's staff used his template, but didn't give Davidson credit. Worse, he says, they use images that are on his server, meaning he has to pay for the bandwidth used from page views on McCain's site.