i-series = consumer = white case design (since 2nd gen iMac & iBook)
:EDIT: and since the iPod is allso a consumer product -> White and with i (exception is the mini, but people think the i-series will evolve in that design direction when next-gen is coming out...)
allso the iSight is an exception since they made it in Aluminium :EDIT:
e-series = education (but became part of the i-series since they sell them to anyone...) = allso white case design
Power-series = professional use wich means more powerfull processors in most of the cases, monitor stretching as an option to only mirroring in the i/e-series, more expandable, better screens on the -books compatible with Cinemadisplays... that kind of stuff... = aluminium design (or Ti for early G4 Pbooks or grey/metallic plastic for G4 desktops)
:EDIT:the old displays were white plastic indeed, but they were designed when the PowerMac desktops were plastic too... :EDIT: