Really strange problem just started on my 2010 mbp running mountain lion.
I reset my machine because I was having problems with outlook crashing (this is not strange well knows anyone who tries to make microsoft products work on a mac), and upon reset for some reason I can't access the web.
The machine appears to connect and obtain an IP address fine on wifi, but no traffic will come through, can't access web pages in any browser, FTP and mail clients don't work. I tried plugging in the Ethernet cable and again the machine appears to connect without any issue, but no traffic will come through.
Other machines in my house are working fine.
Update: I've also since found that if I login on this machine as a guest or as another user, everything works fine... Which seems very odd. Doesn't appear to be a DNS issue.