Craig,
The Context Diagram is and is not the same as the Data Flow Diagram. It is the same regarding the look and feel, and it is not the same in everything else.
You may use a Context Diagram to scope the project as it provides a visual model of the interactions between the system you are going to build and the outside entities that interface with the solution. Such diagram is easily understood by the stakeholders and is used to get agreement on the scope under review.
The system to be build is represented as a circle in the middle and the external entities are represented as rectangles around it. For example the external entities could be Finance, Vendors, Marketing, Customers, etc.
The interactions between the system and the external entities (the processes) are represented as arrowed lines, pointing to the receiving party. The text next to each line briefly describes the interaction: provide promotional details, deliver supplies, request service, etc.
After the context diagram has been agreed upon, you may start with the Data Flow Diagram. You take a single process from the Context Diagram and decompose it in a Data Flow Diagram.
Both Context and Data Flow Diagrams are part of the structured analysis approach.
If you work with Use Cases you may replace the Context Diagram with the Use Case Model of the system and the Data Flow Diagram with a Sequence Diagram.
-Vessela