I have no idea what was broken, but someone figured it out and fixed it and things are back to normal for everyone.
Two days ago I reported how this site was not accessible from AT&T networks. This included AT&T on 3G as well as other AT&T networks, like the one we have in my office. Well, I came into the office this morning to discover that the site was accessible once again. I hadn’t bothered trying from my iPhone, but once it was accessible here I gave it a try via AT&T’s 3G network and it is accessible there, too.