For us publishers will be 68% left when we display the Google Adsense ads on our blogs and websites and 51% when someone searches from our own Google search box.
Google disclosed that large websites have a different revenue share percentage (they earn a bigger amount of cash). This means Google wants to keep good costumers happy with this method.
It was not disclosed if these percentages are averages for every publisher or if there are regional differences. Usually advertisers have to pay a higher amount per click if you select specific countries like USA, Canada, Western Europe instead of worldwide advertisement. With worldwide advertisement the "cheap" pay per click costs are usually generated from Asian and African countries and you'll generate only little traffic from Western countries.
Google will make further disclosure of these percentages directly inside of your Google Adsense account in the following months.
More information can be found in this post from the Google Adsense blog.
What are your thoughts about Google Adsense and its revenue share?