For those of you that might be considering joining the CloudStack developer community, I wanted to gather together a list of the top resources that I've been using to understand the system's architecture and code structure. I consider these an important foundation for any discussion about CloudStack's architectural future, but note that the project is moving quickly to improve the modularization and features. Things are bound to have changed!
Our "developer 101" wiki page is a great way to get an initial understanding of the system internals. It's a quick read, but useful for setting context.
Also on the wiki, the "CloudStack packages and dependencies" page will walk you through the source level relationships (which is important the first time you start digging into the code itself).
Last, some great videos (You should really take a day and watch as much of these as possible!):
Chiradeep Vittal presenting a system-level explaination, helping you with the context for the deployment and runtime environment of CloudStack:
Alex Huang presenting the code structure itself, including currently available extension patterns.