Seriously. It would be nice to see a widget like this that anyone could throw up on their site or blog. Problem would be getting the update -- you'd have to have a server in China pinging all the participating sites, I suppose.
This could be a very nice way for the web to passively protest China's censorship: put a strip at the top of your blog that displays whether or not your blog is readable in China at the moment. Even if the Chinese couldn't see it, plenty of people outside of China would, raising awareness of the problem and putting pressure on China.