There's a very high level diagram toward the end of the Wikipedia page: http://en.wikipedia.org/wiki/XBMC. There are a few relevant notes in the XBMC wiki: http://wiki.xbmc.org/index.php?title=XBMC_for_Linux. I proposed that dev pages be added with a "Dev:" prefix, so if you learn about something in the architecture and want to document it for others (or your future reference), create a Dev:<whatever> page.