The story I linked to includes lots of contributions from readers worldwide about their ability/inability to access the site, their comments, how the blocking works, how to get around it, etc. Quote from the linked page: "It must be hard work keeping up with all those internets."
From Netcraft, who has a story with a bit of an explanation: "Excluding non-U.S. requests could be denying access to the Bush campaign site to some registered voters, including American residents who are living overseas but eligible to vote by absentee ballot." Nice.
[Edit: BBC has picked this story up.]