My visit with MusiChristian.com went well. I spent three days learning more about their business and their existing technical infrastructure, and I was able to offer some general recommendations concerning their future.
I am now writing a technical specification for them, a project that Patrick Reilly and I are collaborating on. There are some interesting new ideas concerning this particular task that try to address the fact that many traditional methods are too difficult for clients to understand. If your client can't make sense of the specification you write, it's less likely to accurately reflect the client's needs and desires, and this can be very problematic during development.
I don't necessarily agree that interface design is a complete substitute for a technical specification. However, I do think it's very beneficial to accompany technical details with example interfaces and to use them as a guide. Not only is this likely to be a more comfortable approach for the client, but it also has the added benefit of forcing you to consider the application from a user's perspective.