There is some evidence suggesting that Google has started to use a dampening filter on new links to websites, so that any new links to a website are not immediately reflected in its PageRank. The theory goes that Google's algorithm causes this delay to discourage wholesale link purchasing. Such a change would require a major rethinking of strategy by everyone who optimizes their websites for the search engines.
Much like the sandbox itself, there is evidence in support of this dampening effect theory. Also like the sandbox theory, there is evidence that the phenomenon doesn’t exist, or is simply one of mistaken identity.
Some webmasters and SEO experts no longer believe that link boost to be the normal course of events. In fact, some experts believe almost the opposite, that the new links are dampened by a filter, and could even cause a temporary drop or hold in the SERPs. Reports also are circulating that Google PageRank is not being fully passed along from new links either.
By dampening the value of new incoming links, Google probably hopes to discourage link sales in particular. By lessening their value, and removing any immediate link boost, Google might reason that website owners will be less inclined to buy incoming links. The problem lies with the possibility that all incoming links, including natural and relevant ones, are being filtered along with the purchased and non-theme related links.
Keep link exchange programs confined to theme relevant sites. Avoid exchanges with websites that have little to no topic relation to your site. Entirely non-relevant links are much more likely to be viewed with suspicion by Google, and possibly filtered. We already are quite certain that Google passes along more of a boost to PageRank and link popularity from theme relevant sites, than from topically unrelated sites.