I just returned from Silicon Valley, having met with a couple of local companies regarding their architectual practices. Two immediate observations from those vists: first, the essential architecture of most of interesting systems really is locked in the tribal memory of a few individuals and second, as such business mature, a focus on architecture becomes increasingly important as a means of driving economies of scale (by harvesting and then exploiting common architectural elements) and innovation (by creating the opportunity for entirely new businesses by opening up their architectures for integration with other systems of systems).
I spent some time with the folks at Adobe who were very gracious hosts. Part of my time was spent with the architect of Photoshop, digging through its architecture. Photoshop is the "professional standard in desktop digital imaging" according to their literature. Opening up the hood, there's a really beautiful and elegant architecture that lies within. The current release of Photoshop consists of about one million SLOC in C++, with several million SLOC of basic libraries written in a variety of langugages. The 80/20 rule seems to apply here: the essence of the Photoshop architecture swirls around abstractions and mechanisms for documents, layers, channels, and tiles (some 20% of the total code) while the rest of the code is there for all the surrounding cruft (licensing, plugins, file manipulation, event handling, the user experience).
I also spent some time with a company who for the moment shall remain nameless, except for me to say that these guys are one of the dot com era's big success stories. They are running with a code base of about five million SLOC in C++ and Java, with a regular rhythm of releases that pushes out a new system very two weeks. It's really a joy to step inside a company where things seem to be clicking: they've got a good business model, they are growing and even hiring, and they are serious about improving their software development practices (which already are far better than the average I see in the industry). Above all, in both this company and Adobe, while there are always issues and risks to be addressed, their development teams seem to be having a good time, and that's always a sign of a healthy organization.