Believe it or not, we are a mere ~18 months from most of the eventual candidates having declared their candidacy (or at least, announcing exploratory committees......which is as good as an announcement of candidacy in most cases). Last time, I think all of the eventual candidates from both parties (except Fred Thompson) had already at least announced exploratory committees by the end of February 2007.
I actually think they will announce thier candidacies even earlier this time, maybe at the end of 2010. 2008 was a milestone in US Election campaigns, with the internet playing quite a big role in it, and the internet will almost certainly be bigger and more used in 2 years time. Does anyone know when Bush announced his intention to run for re-election in 2004, out of interest?
2010 - Football World Cup, UK General Election (probably), my last few months of school, US Midterm Elections.
2011 - Start of US Presidential Election campaign
2012 - US Presidential Election and Primaries, London Olympics, Euro 2012
It's going to be a very exciting three years for me.