The wave of search engines concentrating on personalized search has had me a bit concerned about the effect that will have on search engine optimization. But I suppose it depends upon what the definition of personalization is. The original definition, as I understood it, was to have different results displayed for each user, depending upon that users profile. This would make an SEOs job very difficult. Where does your site rank? Well, it all depends upon which user is searching. Your site might have a million different rankings. It would be impossible to track.
A different type of personalization is happening in some search engines however, that do not seem to affect search results and the positioning of sites. A9.com and Ask Jeeves are both offering new types of personalization services that focus on what the user does with the SERPs after they are presented. They are offering the ability to bookmark and apply sticky notes to favorite results, etc. This kind of personalization is great for the user, but will probably not affect the way an SEO works.
My personal hope is that post-search personalization (as I call it) becomes the favored model rather than pre-search personalization.