The Official F1 site has some pretty good looks at the diffusers:
http://www.formula1.com/news/technical/2009/806/637.html
http://www.formula1.com/news/technical/2009/0/640.html
http://www.formula1.com/news/technical/2009/0/634.html
The McLaren 'cut out' may be an illustration of the rule being talked about. Last year they wouldn't have been able to do that because that area would have had a aerodynamic 'kick up' in front of the rear wheels and becuase there was bodywork above it they wouldn't have been able to sculpt the floor in with the shape of the coke bottle; now the rules have got rid of those kick ups there is no bodywork there hence they can have those cut-outs.
http://www.formula1.com/news/technical/2009/806/637.html
http://www.formula1.com/news/technical/2009/0/640.html
http://www.formula1.com/news/technical/2009/0/634.html
The McLaren 'cut out' may be an illustration of the rule being talked about. Last year they wouldn't have been able to do that because that area would have had a aerodynamic 'kick up' in front of the rear wheels and becuase there was bodywork above it they wouldn't have been able to sculpt the floor in with the shape of the coke bottle; now the rules have got rid of those kick ups there is no bodywork there hence they can have those cut-outs.