I too have been finding my statistics from Google Analytics.
I have done some checking and I am finding that things look to have started around April/May of this year. That is about the time we finished converting most of our pages to the 3.1 template.
Below are some statisitcs from some of our most popular pages looking at a couple months now and a year ago before they were in the 3.1 template.
Then I looked at website I work on and it was has not been coverted to the 3.1 template. This one we are not seeing the 100 bounce rate, zero avg time on page, the same pageview for overal and unique visits
The last couple of days I took one of our frequently view pages and converted it back to the 3.0 template and this seemed to solve the problem.
I am not concerned that maybe all of the numbers have been affected by the templates so I am doing some tests on seeing how many hits are record on similar pages in the 3.0 and the 3.1 templates.
I have also heard this may be happening in the sites that are in the CMS but cannot comfirm this at the moment.