Ever since the first reports of the Google sandbox I've been testing ways to get new domains ranked fast (like it used to be) and I'm 95% certain the Google Sandbox doesn't really exist, the sandbox is a side effect of Google dealing with website owners buying and comment/guestbook/forum spamming for backlinks by changing how long it takes for a new backlink to pass full link benefit**
** Note link benefit is NOT the same as PageRank (PR) reported by the Google toolbar. When PR is updated on the Google toolbar that represents the pages PR calculated roughly one month before the PR value was updated. This means if you add links to a new page today it could show PR roughly a month from now, for example if your new page had a link from a PR7 page with few links from it there is a very good chance your page will be reported as PR6 next update. However this PR will not have aged (the links are new) and so though it says PR6 on the Google toolbar the page will not act like a PR6 page for at least 12 months.
Prior to the first reports of the Google sandbox it was possible to buy a domain, add a few hundred links (bought/natural or otherwise) and with OK on page optimization have it competing with older sites with similar backlinks in a few months. For example I managed to get my very first website banned for link spamming (got it to PR7 home page and 8,000 unique visitors a day), I started a similar site after realising it was banned and in 3 months was already at over 3,000 visitors a day with only linking from my own free sites (Geocities etc..., was a new to SEO and very cheap) and one other domain I owned, did no link spamming (so didn't have that many backlinks).
That is no longer possible because links no longer pass their full benefit within a few months, instead what would take 3 months or so now takes at least a year and if you haven't gained a reasonable number of links in that time significantly more than a year!
This makes sense for Google, imagine trying to filter billions of keywords with a formula based around some arbitrary measure of competitiveness when it can be automated. Google has said they prefer to automate their search engine algorithm as much as possible.
Google gains in the following ways:
Buying Links Costs Too Much
Website owner buys a three hundred text links for at least $5 a link ($1,500), he believes this will result in significantly increased search engine rankings and has budgeted for 6 months on the understanding he'd see results within the 6 month period ($9,000).
3 months on the website owner is down $4,500 with absolutely no improvements and starts to consider if this was a good investment after all.
6 months pass on, now down $9,000 with out significant improvements, prior to the Sandbox effect a website by now would be seeing significant improvements that would encourage the site owner and might even result in a further purchase of links. With today's Google a business buying links is going to have to hold off significantly more than 6 months (a year really) and I don't know many business owners who would wait a year with no signs of improvements half way through the year.
Backlink Spamming No Longer Works
With link spammers it works out just as good for Google. The vast majority of web pages that can be spammed for backlinks: guestbooks, forums and blogs over time are moved further and further away from the home page meaning very little link benefit gets to the page and so very little is passed through the spammed links.
For example a link spammer finds a nice PR5 page they can gain a follow link from on a blog. They add a bunch of spammed links and think they are going to benefit from PR5 backlinks. A few weeks pass by and the blog owner has posted several posts a week and at the next PR update the once PR5 page drops to PR5. OK, PR4 backlinks isn't so bad. Six months later however our prolific blog poster has posted dozens of new posts and the once PR5 page is so far away from the home page it's now PR2. Add to this every link spammer and their dog has found this blog page and has also spammed it for links and the amount of PR/link benefit passed is negligible. A year on and it's even worse, that link is practically worthless.
Google is constantly looking for sites gaining links through comment spamming etc... and there's a very good chance if a link spammer, spams for links prolifically (which they have to, to accumulate significant link benefit) there's a very good chance within the year it takes for the link benefit to pass from the first batch of spammed links Google's tagged the site as one using blackhat SEO techniques and has banned it from it's search engine. When I link spammed my first site and got it to PR7 it didn't take Google one year to find and ban it and this was before the sandbox effect was reported, so had they devalued backlinks then as they do now in the first year I'd have not benefited at all by comment spamming!
For anyone considering link spamming, like I say above I used to use that blackhat technique. I have no major problem using blackhat SEO techniques that are risk free, but because of my experience of using link spamming I no longer go full blackhat, it's not worth the risk (I go grey from time to time :)). The site I got banned for example sold £80,000 worth of stock because I link spammed, (well might have sold £30K without the links) but over the last 6-8 years since it was banned the site hasn't made a penny from selling products (it has an affiliate site now that probably makes $100 a year!). Had I never link spammed I'd have made far less that first year, but I also built legitimate backlinks and I'm quite good gaining backlinks without using link spamming, so by now I could own a website that pulls in tens of thousands of visitors a day (I got it to 8,000 a day with link spamming) and be making hundreds of thousands of pounds a year.
Basically link spamming is not a good idea long term, if your site does really well from link spamming you'll have dozens of competitors reporting your actions to Google and before you can benefit significantly from your actions your site will be banned permanently!