Bob,
Here's some explanation about the old and new package structure. I don't think there's any reason why packages from ACS3 couldn't be ported, they just don't work as they are because of the fundamental changes in the system.
If you want to learn more, you might want to read Reuven Lerner's articles (LJ 101-105) about OpenACS and/or go through the problem sets.
OpenACS, like any system of its kind, is big and you probably can't get the big picture by just looking the code. You can't get OpenACS without any packages, since everything is organized into them. You can get the bare minimum of OpenACS by checking it out with CVS with module name "acs-core" - see CVS instructions for how to do this.
