Using the cable channel for identification is quite common, especially among stations with high-UHF assignments and in markets with high cable penetration and consistent placement, either from one dominant provider, or all of the major providers placing the station on the same channel. KASW 61 Phoenix has identified as channel 6 for some time now, first as WB 6/61, then as WB 6, and now as CW 6.
However, that is not the legal identification, as the legal ID must include the call letters and city of license.
At this time, digital stations use their analog channel for their PSIP ID, but I wonder if, once the transition phase is completed, that stations will eventually be able to identify themselves by their cable channel in their PSIP ID, provided of course that the channel number is not already being used by an over-the-air station.