Google Instant was launched on September 8th, 2010. Aside from saving the Google Instant searches that had already appeared and disabling them from reappearing in subsequent Google results, the algorithm itself that enables Google to index websites has not changed much per se:
Google's "Search Engine Optimization Starter Guide" was last updated on November 13th, 2008.
Google Instant does not leave SEO indifferent though. Google SEO professionals are on their guards, as they realize how versatile Google can be. The fear should stem from somewhere else; from the way users will interact with Google thereon. Something, I am certain, not even Google, can predict.
Had the users remained indifferent to Google Instant's suggestions, and the results parading on their screens as they typed, Google Instant would eventually have no effect on SEO. Yet the conflict lies here. Are people likely to change their intended keywords or get distracted with given results before finishing their search terms? I would say yes. Google users behavior shift with Google Instant.
What would change in SEO?
Not much when it comes to ranking high for given keywords. Yet, it would greatly affect users' keyword choice, as keyword suggestions, are bound to offer Google results... instantly. Giving shorter keywords an advantage as to appearing more often than not on the screen before users get distracted with suggestions.
Instead of basing your keyword choice on Google Adword's measures, from now on, you need to consider the probability of another relevant keyword showing up before your terms are typed.
Leaving aside Google's personalized search results, which I would assume only apply to a few search terms of interest, results will be based on general Google probabilies. I am, thus, lead to believe that Google Instant would eventually have a sociological effect, converging search results to the same keywords, and consequently to the same websites... to the same information displayed.
An opportunity for SEO professionals to gain traffic exponentially by optimizing keyword choice?