This can be done in many ways, and with more recent versions (2007 and 2010) you can link data into a Visio diagram without writing any code:
Visio/Excel data link
If you're working pre-2007 Visio or with PPT, then I think you'd want to use VBA to create the link. The problem then becomes how do you persist a link between an Excel cell and a Visio/PPT shape.
I've worked with a tool called eVSM that dumps a process chart in Visio out to an Excel table. In order to maintain a persistent link between the Excel worksheet and the Visio shapes, they store the GUID of a shape in the Excel cell comment for the shapes cell. You might be able to do something similar if you can get a unique ID for a PowerPoint shape.
This update mechanism gets run manually because it is relatively time intensive, so it doesn't work like in Excel where you modify the value and the linked display changes instantly, so you'd have to think through how you want to accomplish that.