Over on his blog Mey has published a diagram that gives an overview of the EP 2009 architecture. I already talked about parts of the architecture like the ASP.NET User Control, the AxDataSource, etc. here. By the way Mey is the Program Manager for EP and Role Centers. Right now he is in sunny Orlando, Florida (the weather is pretty bad in Redmond right now) for TechEd 2008 where he is presenting sessions on EP and Role Centers.