Google makes it clear that it disapproves of certain SEO tactics, such as showing one page to Google’s spider and another to actual human visitors.
A doorway page is a page built specifically for the purpose of ranking well in the search engines and then links to the “real” destination page, or automatically redirects there. Doorway pages are a popular choice of some SEO firms, although Google has cracked down on this and many webmasters saw their pages disappear from the index. Some SEO firms call their doorway pages something else, in an effort to convince the customers that it is the right tactic. Most of the major engines may decide that an orphaned page is a doorway page, and if so then the page or the site might suffer a penalty.
For more information on the topic of doorway pages and Google’s policy regarding this black hat SEO tactics, please visit:http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66355