Every request that comes into Google's search is sent to a specific datacenter where the results are returned. As of today, there are more than 50 unique IP addresses known to the SEO world that are returning slight variations of search results. Although many ponder the reason for this, clearly it is something that has been a part of Google's search technology and has no reason to dissappear in the near future.

However, Google's use of datacenters can sometimes give away changes that may be about to happen in the search results before they are found worldwide. Many SEOs monitor the datacenters daily, and SEOmoz is no different (just see our rank check tool - allowing users to search 30+ datacenters at a time). Lately there have been several uproars at the forums over different results from specific datacenters and their chances of propogating to the Google search network as a whole.

Google has certainly done this in the past - making slight alterations and testing them out at a public datacenter, then shifting the algorithm changes over to the entire system. One good way to see if the results are likely to propogate is to wait for more than one datacenter to change to the new results. If you can find 2, 3 or more datacenters shifting to a new set of SERPs, chances are good that the change could become permanent. Remember, if you do see these changes, please report them.