My brother's house has 3, yes, three digital devices, 2 digital cable boxes and 1 modem.
The digital cable boxes function very much like a cable modem with a return band below 50MHz and QAM modulation for the downstream channel. (In fact, you'll find that as the cable modem boots up, it stops on all of the digital cable data carriers, checking to see if they're the downstream for the cable modem).
Problems which affect the cable modem WILL affect the digital cable boxes.
