While this code is still somewhat crude and fails for a small percentage of visitors, it appears that repeated high-performance cache sniffing is a viable possibility. The approach should allow several hundred URLs to be tested per second without disrupting the cache or causing other side effects. For more, check out these links:
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PS. You may also want to check out a "competing" variant inspired by my original post - see here. The author is leveraging image timing for Firefox, and noticing that in WebKit, image navigation can be aborted with window.stop(). His code is fairly sensitive to machine performance and does not work for me very reliably, but it's a good alternative in non-MSIE browsers.
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