I've tried this:
cd /Library/Preferences/SystemConfiguration/
sudo mv preferences.plist preferences.plist.sav
and I still get the same prob. No yahoo.com, no google.com or gmail and no youtube....it's my gmail that concerns me. fortunately i have a pc, on which all of these sites still work. in talking w.mac, they say it's a comcast issue. comcast are morons and are sending someone out on tues to look at things here.