The problem of blogs, published to a custom domain, suddenly returning a "404 Not Found" when access is attempted, has been chronic since November 2007. Almost daily, we'd see reports of 2 or 3 such problems.
This evening, we saw 4 reports in a space of 1/2 hour. And we continue to see more.
The good news is, so far the reported cases all respond favourably to having the blog published back to BlogSpot, then republished to the "www" alias, using "Advanced Settings". In some cases, the victims have reported success after repeating the republishing cycle.
>> (Update: 5/18): Blogger Support is now suspecting a link between use of Google Apps, and the 404. If you are having this problem, please help us diagnose the problem. Describe your use of the following Blogger / Google applications / wizards, with any possible details (relevant dates would be useful), in setting up or maintaining your domain:
>> Forum thread links: bX-*00076
>> Copy this tag: bX-*00076