An A320 Captain took umbrage at being cleared for the CADIT RNAV SID from Runway 26L at ATL and; when cleared for takeoff; was cleared to fly a 295 heading after the marker.

Narrative

[We] received CADIT RNAV departure on our PDC. Informed Ground Control that we were assigned and expecting that departure. We carefully reviewed the NOTAMS; ATIS; departure page; 10-7 page; and the 10-3 page that describes the RNAV procedures at ATL. AFTER we were in position on the runway; the Tower cleared us for take off and added: 'Fly heading 295 at the marker.' We asked him to clarify and he just repeated the same thing. I asked him did he mean one of the markers for 8R. He said 'yes; the middle marker.' I informed him that we were assigned the CADIT RNAV. His answer was that they were doing triple parallel departures. We turned up our marker beacon audio (there are NO marker beacon lights on the Airbus) and departed. After takeoff I again reviewed all our paperwork and there is NO MENTION ANYWHERE OF THIS MARKER BEACON THING! One note on the 10-3 page says that if triple parallel departures are being conducted to expect a HEADING; NOTHING about a marker. If this is a standard departure; why isn't it in the 10-7 page or somewhere? It should be on the ATIS at least. Why didn't Ground Control warn us that we could expect it? We don't use the marker beacon audio on the Airbus so how do we know if it's going to work? Had we had some warning we could have set something up to identify the point.