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Considering that guestbooks typically have low PR and tons of links, I'd have to think it would be a waste of time if anyone thought signing a lot could get them anywhere. One ODP link is likely worth more than a couple hundred guestbook ones. Also, Google would be way off base assuming that anyone who ended up signing a lot of guestbooks did so with search engines in mind. I know one bored woman who enjoys surfing around to just various personal sites, and always signs guestbooks when she finds them. She has no interest in search engines at all. She just likes signing guestbooks. Google also fails to consider that those who place URLs in guestbooks hoping for traffic are doing so for reasons totally other than to boost SE rankings. What if Jane who sells widgets at widgetworld.com signs all guestbooks she finds on the theory people will follow that link and maybe buy some widgets from her? She isn't even considering spiders, but is aiming at the human surfers. She just figures if someone wants to put up a guestbook where she can advertise for free, why the heck not? Ain't no law against signing website guestbooks.
Months ago, somebody put my URL--with phony e-mail addresses from my domain--in a large number of guestbooks that were completely unrelated to my topic. It doesn't seem to have hurt me at all in Google. (I've even had a few referrals from those guestbooks. One Webmaster even wrote me to compliment me on my site, amazingly enough!)