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Checking the color of <a> gives similar information. It's all client-side so you can do 40k+ URIs per second.

Here's code I wrote to display the "digg this" button only to digg users: http://int2e.com/blog/improved-digg-integration-script/

Unfortunately, the next version Firefox will block this hole, and I imagine other browsers will follow suit.

Why is that unfortunate?

I use visited links to personalize my site.

There are definitely privacy implications when doing it on a large scale, but I wish there was a middle ground.