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GG has stated several times that getting "organic" links is preferrable to "other" methods.
I would say a site that garnered 5000 links in a couple of days would not be the best example of "organic" link generation.
GoogleBombing is what bloggers(and other people) do mostly for personal fame. In the community they exchange linking to each other's sites with certain keywords in the link's anchor text. If a particular John Smith wants to be the #1 result for the search term John Smith he will get his friends to link to his weblog as John Smith. This can be very rapid and very succesful but since they are Blogs... usually not permanent.
There was an article about this in the wall street journal last monday I think where they gave the example that the search term "God" returns PHPNuke as #2 result even tho it has nothing to do with God, they just had people link to them with God in the anchor text.
In a keyphrase I monitor, I saw a site rise from nowhere to top 10 for about 10 days using this technique.
I haven't seen the site in months. Coincidence? I don't think so.
There are really only 2 explanations:
1) G "spanked" them
2) So many other websites spammed the same guestbooks that their links were rendered useless.
Proceed with caution. If you try it - do it with a site you plan on throwing aways in a couple of months.