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The way it is supposed to work is that Google doesn't penalize for links from FFA pages, link farms, or guestbooks unless you are also linking *to* them. If Google is penalizing for this, it would be trivial for a competitor to just pay some teenage nephew who will work cheap to sign up his competition to a few hundred FFA pages and guestbooks so that he can get his competition thrown out of Google. In the sometimes cut throat world of e-commerce, such could be expected from competitors if Google penalized for such. This sort of thing could even happen easily with non-commercial sites. I've been e-mailed bombed a number of times by people who I've flamed on Google. People will do all sorts of things on the Internet for the old fashioned reasons of spite, malice, and revenge. :(
For what its worth, even if Google doesn't penalize for links from FFA and guestbooks, I doubt adding links there would be worth your effort. Perhaps there is somewhere lurking out there a guestbook that has a PR8 and only has a few links on it that adding your link to it might do you some good. However, seems to me all the guestbooks that exist in reality have a really low PR, and that is diluted by the fact those guestbooks all have a ton of links. A link on such would be near worthless. Just one DMOZ link should be worth many hundreds of guestbook links. And, if you check around you may have friends with home pages that have managed to get a PR5 or better (I've seen quite a few), and just asking the favor of one link from that page would be a lot more effective than adding your site to a thousand FFA, etc. pages. Far better to work on just getting a few links from pages with decent PR, and spend your time creating good content for your site that will get you even more links. The importance of PR tends to be *way* overblown. All in all, traditional techniques of creating good content and getting relevant links is far more important.