It is classed by spamming in some engines, and is therefore a *bad* idea...it is simply not worth the risk, especially when you consider that search technology is only getting more advanced and better at weeding out such "cheating."
Just a quick one. I have heard it said that if you put hidden words and groups of words in your site, ie by putting in some text that is the same colour as your background, that this will increase your chances of getting a listing. I already know that repeating words in your meta tags that are in the main body of your text works well. But is hiding and repeating words classed as spamming the search engines?