Just a quick question. During CD installation, a point is reached where a choice is offered to mount the CD and install packages from there or to install from a previously mounted source medium. One assumes from the very presence of this latter choice that that /usr/src would need to be mounted at that point via one of the virtual terminals, say ALT-F1. Would the user then be able to build the base system from source at that point? I'm unclear on this point.
No, this option has nothing to do with building from source. It's simply a way for users to manually mount the media containing packages, which is useful if your media is not a CD-ROM.
For example, if you had a flash drive with a bunch of packages on it, you would have to head to vc2 and manually mount it under /src, so the setup script can find the packages.