Google Analytics (GA) announced it changed its definition of a visit (or session). This will necessarily have an impact on your metrics in GA, and at this moment in time, it is not clear if the changes observed are the result of a bug in the new implementation, or the new normal... We are following closely and will keep you posted as things develop.
UPDATE: (08/15/2011) A very good summary of the impacts can be found here on the L3 Analytics blog.
UPDATE: (08/18/2011) According to Google's blog post, an issue was identified in the new visit count code and was fixed on Tuesday, August 16th... Initial reports seem to confirm this - hope you are seeing too!