Personalized search is the engine [that] is going to drive universal search.Gord makes a good point. When talking about universal search, Marissa Mayer motivated it by saying, "The best answer is still the best answer."
When you look at the wording the Google throws around about the on-the-fly ranking of content from all the sources for Universal Search, that's exactly the same the wording they use for the personalization algorithm.
[Google's personalized search] operates on-the-fly, looks at the content in the Google index and re-ranks it according to be perceived intent of the user, based on search history, Web history and other signals. It's not a huge stretch to extend that same real-time categorization of content across all of Google's information silos.
As Google gains more confidence in disambiguating user intent, more specific types of search results, extending beyond Web results, will get included on the results page and presented to the user.
But, the best answer for me is not necessarily the best answer for you. Especially in cases where there is ambiguity in intent and multiple possible verticals that might be relevant, the deciding factor could be what I have done in the past, my search and web history.
If Google is not personalizing universal search already, it likely is only a matter of time before they start.