Digital Photography for Standard Cameras?

NightWhistler asks: "I've been hearing stories for some time now about digital modules that can be used inside normal photo camera's as a sort of 'digital film', effectively turning a standard camera into a digital one. If they exist and performance is good, I would love to get my hands on one of those babies... ;-) Has anyone actually seen one of these, or perhaps have experience with them?" There may have been one company that did this, but I think they went out of business, recently. I've always thought this was a neat idea, but is there really a market for this kind of modification?