Doorway Pages: Good SEO or Black Hat SEO?

Doorway pages fall under the general heading of Black Hat SEO, otherwise known as "Easy Ways to Get Your Site Banned, An Un-professional Repuation, Or Both." (didn't I see that on an informercial somewhere?)

What are doorway pages? Let's check.

• Doorways are simple HTML pages that are customized to a few particular keywords or phrases. Sounds okay so far.
• They are programmed to be visible only by specific search engines and their spiders. Huh. I'm getting bad vibes here.
• These pages are designed for one purpose and one purpose only: to trick the search engines into giving them higher rankings. Warning! Warning! Danger, Will Robinson!!