I think it is generally a bad idea to try to trick search engines into ranking your site higher in the search results since if you stuff your websites full of keywords, and invisible text then the search engines can ban you from their site. Plus, it makes it tougher for people to look for quality content on google, and yahoo if all the crappy sites are rank high.
I seen it a few times, but I forgot the links to the page.
I think spambots who spam blogs and forum with their links hope to get a higher search ranking because they create a lot of one-way links to try to trick the search engines into thinking they have a lot of legitimate people linking to them through Social sites, guestbooks, blogs and forums.