Many sites, including google.com, were inaccessible yesterday in the US because of a DNS failure at Comcast. A Google spokesman said that "Google engineers helped troubleshoot the problem and provided diagnostic information to the ISP. We believe the issue has since been resolved by the ISP."
PC World reports that the problems affected people from the northeastern part of the U.S. It's interesting to note that localized Google sites like google.co.uk were accessible.