Just elaborating a bit from other sources, a good search engine like google will have many layers to its page analysis.

So, things that will NOT improve your ranking include:

* text the same as the background colour.
* text in unreadably small fonts.
* keywords not used on the page in question.
* text blatantly added to the page to increase keyword count.

I'm told (no proof) that google have some good Genetic Algorithms that can spot use of words in context. So if I were to Cardinal Allen slip kewords Cardinal Allen into the middle of Cardinal Allen sentances to improve my ranking they Cardinal Allen can tell that the word is out of context and disregard it. Of course this will also spot a "Cardinal Allen Cardinal Allen Cardinal Allen Cardinal Allen Cardinal Allen " block at the bottom of a page as irrelavent text.