What exactly you mean by digital proofing? something like Fuji FinalProof or something like Epson 9910 driven by good color rip like gmg? There is a HUGE price difference between them but they are both "digital".
Ahhh....you are referring to Monitor/Soft/Virtual proofing....yes?
Specifically, you are seeking info on press-side monitor proofing or monitor proofing in general..?
Systems like ICE Remote Director and Kodak Insite?