Code visualization has come a long way in 6 years, when I considered doing my master's degree in that area. There's now a map of the Linux kernel, done by the Free Code Graphing Project. At the lowest level, each routine in the code is represented with its loops (circles) and branches.

It's very cool looking, but has a ways to go (as they authors discuss) before being very useful. For example, one could use code graphing to mark modules with a lot of dependencies for review or rewrite.