The following documents give a high-level discussion of some aspects of the ORB architecture:
Here are some more detailed descriptions of parts of the ORB design:
There's not a lot written for this layer either. There is a lot of code in the ORB that represents the GIOP protocol messages, but that is pretty simple. The transport-protocol dispatch does need to be documented somewhere.
The ORB also has an extensive internal SPI that is used by the GlassFIsh project. Javadocs for the ORB SPI are available here.