[Marco van Herwaarden]

By presenting them a different page then a normal guest, this is called cloaking.

An example from the google FAQ:

Alternately, your page may have been manually removed from our index if it didn't conform with the quality standards necessary to assign accurate PageRank. We won't comment on the individual reasons a page was removed, and we don't offer an exhaustive list of practices that can cause removal. However, certain actions such as cloaking, writing text that can be seen by search engines but not by users, or setting up pages/links with the sole purpose of fooling search engines may result in permanent removal from our index. You may want to review our Quality Guidelines for more guidance. If you think your site may fall into this category, you might try "cleaning up" the page and contacting us with a re-inclusion request. We don't make any guarantees about if or when we'll re-include your site