Found this very good video on Andrew Price’s site. It deals with composition, not only in photography (Andrew is a user of Blender) but also in video or anything else which requires a graphical output.
As he states himself, it’s much more than just the rule of thirds. So if you have a spare half hour to burn, check it out either on his site (linked above), or on Youtube. I found it via the.me, and thanks to Daniel for linking to it!