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I don't see this as being a healthy long-term strategy, my guess is Google will do something about this.
Short answer : it currently works and works well, use at your own risk.
There have been a couple of recent threads from people wishing they could get their url's OUT of guestbooks that they have signed. It ain't easy.
It may not be the case now, but at some stage Google will put some type of penalty or ignore the links from GB's, lets face it it’s got to happen considering how it’s been discussed here as a route to get better rankings and cheat the PR system