Funny, rozallin and I were chatting about this just the other day...

At the very least the Safari system (the only one I know of) does "catch" people who are perfectly innocent. They send you mysterious messages:

This e-mail is to notify you that an unusual usage alert has been triggered on your Safari Tech Books Online account. Safari does this to protect your account in the event it is being compromised and to protect Safari's intellectual property. Please see Safari's Terms of Service. Continued alerts will result in the system locking out your account.

Please make sure that you are not running programs that 'speed-up' web browsing; or that spider, crawl, or capture web pages for offline viewing. These types of tools will trigger further alerts and violate the Safari Terms of Service.

Thank you, The Technical Support Team Safari Tech Books Online

They appear to do this by logging regular and repetitive page downloads. I know people who have downloaded tons of stuff off of their service by setting their spider to download at random intervals.

Others have been caught by the Safari service as well. I found the following links here and here (two posts by the same author) to be very informative. And I can confirm zentara's post as a situation that has occurred with at least two Safari customers.

If you are considering signing up for the service, make sure you read the Terms of Service carefully. If you teach, for example, there are limits to how many chapters you can reproduce for your students. That can seriously reduce the total value of the service to you.

An advantage that has yet to be mentioned (at least explicitly) is for people who live in places where these books are not easy to find (like a lot of Eastern Europe). But are these same people likely to have the kind of internet connection necessary to get the most out of the service?

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Allolex