I'm sure there's a valid historical tradition of potential candidates for public office signifying their intent by throwing their hats into some sort of ring, but these days things are done a little differently.
Nowadays a candidate usually lets their intentions be known by accidently posting a revised web page on their official website, or they tell a reporter about it at a public event, or they post a Youtube video of themselves on the Internet.
John Edwards has done all three. From johnedwards.com:
He's the first major Democratic candidate to declare in what promises to be an exciting 2008 election cycle.
Hey, it's only just over 12 months until the Iowa caucuses...a good showing there can really launch a candidate toward the White House.