From google's spam report page:

Don't deceive your users or present different content to search engines than you display to users, which is commonly referred to as "cloaking."

Cloaking is a bannable offense, and indeed if you do it you're certain to get banned sooner or later.

With a couple of exceptions. "Registration required" websites like NYT and online journals and (conference proceedings) like SpringerLink, IEEE Xplore, the ACM portal etc. that require payment seem to be able to cloak with impunity. When they see googlebot, they open up their content to it, and google happily indexes it. So you search for "how to save the world," and google gives you a result from one of these sites like with the "ten easy steps to save the world." You're delighted. You click on the link, and are greeted with "please pay $$$ to see this article."

"But wait a second," I hear you say. That isn't really the same thing as spam because the content is the same, it's just restricted access. No difference, I contend. From the point of view of 99.99% of the users who aren't going to pay, it's an equal amount of annoyance. (Normally I'd be able to access Springer and ACM content at least from school with the IP-address-detection thingy, but now that's stopped working too, and I'm too lazy to figure out why. But that's beside the point.)

It'd be in google's self-interest to not piss off its users and remove paid content from its database. In any case, I think cloaking in any form is disgusting and unethical and I find it surprising that supposedly reputable publishers do it and don't draw any comment. I guess the only good thing about it is that at least in CS the whole print medium is going the way of the dodo, and soon all publications will be available on citeseer/arXiv/author homepages/etc.

Thoughts?