Had the same problem at work. It was our DNS servers, we just cleared the chache for Yahoo.com and Yahoo.ca and it worked again. We don't use Shaw as our internet provider either, so it isn't just a Shaw problem. Yahoo probably made a change and as a result some people will have stale records in their DNS servers, thus people who rely on those servers will not get to yahoo.
Also, I'm a Shaw user and yahoo loads up for me so... neener neener neeener!
