Following is youtube link which explains the BW architecture in very efficient way. The video needs to be appreciated in the way it captured the wholestic view and interaction between various sections in BW throughout version 3.5 , 7.0 .  Awaiting video on 7.3

http://www.youtube.com/watch?v=fovmnSIplpc ( BW 3.5 architecture)

http://www.youtube.com/watch?v=dCcyKmdwVYk (BW 7.0 archetecture)