The same happened to our site. But also Google Analytics shows quite different results : bounce rate, time on site, keywords. All have dramatically changed, so maybe it has more to do with the Google Analytics set up that with real more visitors.
Basically, in light and along the lines of the recent "improvements" Google screwed up another good thing by rendering Google Analytics unusable.
Whatever session ID "improvement" they were after failed miserably.
It is so much fun watching analytics now with a single session being chopped up into multiple visits. It was truly entertaining to find out that our site had 17 visits from google organic visitors today using a fairly common (sarcasm) keyword "one sort of widgets in one specific area stores -competitor widget term" (visitor punctuation retained).
Our saite-wide bounce went up from 24% to almost 80% in the recent few days.
New vs. Returning visitors is proportionately affected as in the example above GA reports 1 new visitor and 16 returning visitors each looking at 1 page per visit, all coming through the same source, having the same browser and resolution coming through the same network and all located in one small town with a population of under 5000, with a total time on site 00:00:00 (funny... according to internal logs this particular visitor looked though 17 pages within a total session length of just under 8 minutes).
This is not a single example. Most of our visitors are affected in the same fashion.
For those of you who is interested here is the cheerful announcement by Google staff regarding this issue. Comments are hilarious. [analytics.blogspot.com
Anyway. Closing my GA account today and moving to another online service.
I've had it. If they use GA stats for SERP (and thy probably do) GA is simply dangerous now. :)