A thread over at Cre8asite forums named New kind of spider is in town links to a Wired article named Covert Crawler Descends on Web. In short, this article describes a new kind of spider designed to crawl the Web as human-like as possible.
How Does it work?
Barry Welford calls this spider, "somewhat scary" and that I agree with. Ron Carnell has it right, "any robot that doesn't ask for and then follow robots.txt is, by definition, unethical." So Ron gives you a technique you can use to track and then block this type of bot.
Forum discussion at Cre8asite Forums.