It turns out that it was due to an URL change on the usgs.gov server, and the widget framework's XMLHttpRequest object doesn't seem to automatically follow the redirect. So I changed the link and it all seems to work again. I sent a new widget to the people who had emailed me to make sure that it solves their problem, and if it's all okay then I'll submit it to Yahoo! again (they approve widgets manually, so it pays to make sure it's right before submitting).
If you're looking for the latest version (1.5) before it gets submitted to Yahoo!, see the Earthquakes Widget page which has a direct download link.
I've uploaded the quake-widget repository to GitHub.