What's the Impact for SEOs?

• Rank Checking is Less Universally Accurate
  While not the biggest tragedy, it's certainly a bit frustrating to know that rank tracking (manually or with tools) may provide somewhat less authoritative data than before. Though, to be honest, rank tracking has always been about establishing a baseline, not about exact results (see previous posts on this). Still, if you've been using this data to see how you fluctuate in the "normal" (non-personalized or geo-targeted) results, it's still solid for that purpose and may actually help you determine if you're gaining or losing in the new, personalized world (if you get more traffic but rankings stay the same, personalized might be helping; if you gain rankings but don't proportionally benefit in search traffic, it may be hurting).
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• The Rich Get (Even) Richer
  Those at the top of the results, who "own" the queries around their niches are likely to benefit disproportionately as mid and long tail queries that would once have shown more alternative sources will now bring up those "previously visited" sites even if their traditional relevance and popularity scores wouldn't have earned them a top position. This will likely contribute to some lowered diversity in the results, but may help fight against low quality re-publishers and content aggregators in favor of trusted brands.
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• User Experience & Branding Boost SEO (Even More)
  It's always been critical to make users love your site, but now the direct SEO impact can be felt even more strongly. Sites and brands that "suck at SEO" may even find themselves performing better if their users love them and the pages are, at least, accessible to engines. I'm buying Steve Krug's new book - Rocket Surgery Made Easy - ASAP either way :-)
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• Buying Traffic May Now Help Organic Results
  If Google really is using signals from all sources of data, the paid results and their impact on search and visit history might now give a boost (indirectly) to positioning in the organic results. In fact, it could be that even services like Google AdSense or other paid advertising that leads a visitor who's logged in to their Google account and using the toolbar (or other detectable methods of tracking) will "count" towards the personalization metrics. I expect lots of SEOs to start testing and reporting on this soon.
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What Should We Do Differently in our Campaigns?

• Get More Visitors (Any Way We Can)
  Depending on how Google is counting visits and traffic (which they haven't and probably won't ever fully disclose), any way you can drag a visitor to your site and give them a good experience is likely to positively contribute to your chances of ranking better in personalized results.
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• Improve Brand Loyalty
  SEOs haven't classically focused on brand metrics and branding as a marketing practice, but it's long past due. The benefits of building a strong brand are evident everywhere in the consumer (and B2B) marketplace. Now Google's giving us one more reason (and a more direct one at that) to start earning visitors' love and, in turn, be rewarded by higher rankings. 
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• More Tightly Integrate Metrics w/ Rank Tracking
  Again, this has been a wise move long before personalization, but with the expansion comes renewed need for weaving together the 3rd-party tracking of rankings with the traffic metrics from your analytics to provide a full picture of how your site is performing in the search engines.
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The big takeaway here is that these action items aren't particularly groundbreaking. We should have been doing all of these as responsible, effective Internet marketers anyway.

Is this a Major, Tectonic Shift in SEO?

No. I'm maintaining my previous stance that unless a shift from Google fundamentally changes the classic SEO process:

1. Make pages accessible
2. Target with keywords that searchers employ
3. Build content that users will find useful and valuable
4. Earn editorial links from good sources

It doesn't qualify as a "tectonic" or "massive" or "fundamental" change in SEO. The best practices we've been recommending to clients, developers and content creators for the last half-decade are actually less impacted by this change than by some of the other items we've encountered recently (Bing + Yahoo! combining, real-time results at the top of query results, more vertical results in the SERPs, etc.). These latter examples call for much more active changes, learnings and direct action on the part of SEOs vs. personalization, which by-and-large just strengthens the reasons for best practices we've long known to exist.

p.s. Tomorrow evening at 6pm (Tuesday Dec. 22nd), I'll be attending an informal SEO meetup in San Diego, CA at the Gordon Biersch Brewery in Mission Valley - 5010 Mission Center Road San Diego, CA 92108. Hope to see some of you there before the holidays!