In March 1º Google will implement the new policy. This consists in the integration of all google products that will be more accurate, more personalized and much more user oriented.
This step was made for the interest of user, and try to find what is more relevant for then (rumors saying that they will actualy tell you if you'r late for work based on you location and your calendar...).
Since all the Google products will be seen as one, your searches in Google will reflect the searches on youtube. So if you search for [jaguar] on Google the results are basicaly the same as if you search [jaguar] on you tube. All this happend if you'r logged on your Google account.
Does this affect SEO? Well, for my point of view...yes.
First, we must not be logged on for our results be more precise and don't "mining" the SERP's.
Second, user's don't tend to refine refine searches very much, they usualy stick to what firts pages gets, so more useful pages (and with a good optimization) may be left behind.
Third, when you search for some restaurante you can get information from a friend ( Goolge +) first and not the actual restaurante.
Most of the personalized content are here since 2005, now they introduce (Google +) on it and make you one giant client for them to study.
One other aspect is the "remarketing" of ADS. Goole can advertise you even when you'r in a non-Google website and not log on. This isn't based on what you search for but with a anonymous cookie tagged on an advertising site.
So, what does it mean?. If i search [buy car] in Google, you can start to see more Ads related to your search even if you'r not on a Google site but sites that carry Google Ads.
Finding the "opt-out" for this is no easy task but you can find it here.
There's a disable button too for Google + for them not to take relevance on your searches here.
So, if you don't bother with none of that, you will see more and more friends on search results...or simply.."unfriend" them.