Distracted by pressurization problems stemming from MEL'd pack anomalies and concerns over the late advisory of the impending landing to the flight attendants; a B767 Captain failed to call for the appropriate flap setting as he slowed for approach; causing the alpha (Angle of Attack) bug to display until thrust was added and the flaps extended to the appropriate setting.

Narrative

Aircraft had two mechanical discrepancies signed off by maintenance. The First Officer reviewed the logbook; and aircraft was legal to fly. After push back from the gate; we discovered that upon further review; one pressurization 'pack' would probably be unavailable for use until an altitude around 20;000 feet due to temperature issues within the pack. This left one pack available for pressurization.Flight was uneventful until descent into Atlanta; where we discovered that the other mechanical write up affected the remaining pack we were utilizing for pressurization after securing the pack affected by higher ambient temperatures in the descent. The remaining pack would not stay operating with power retarded for the descent. The workload on the descent became very high while attempting to comply with Center's requests; and keep the aircraft pressurized. Consequently; at 10;000 FT AGL I failed to inform the flight attendants to prepare the cabin for landing. The Lead Flight Attendant realized we were very low; and contacted us at about 5;000 FT AGL while on a left base for an ILS PRM 28 approach to ATL airport. I told her to prepare the cabin. This was my--the Captain's--flight leg. Workload was still high in the cockpit; and I was further distracted by the worry concerning the flight attendants being able to stow the carts and sit down before landing. I almost asked the First Officer to call them back and make sure we were safe for continuing the approach.At this time were approaching the outer marker on the approach and I began to slow the aircraft to the Flaps 30 approach speed while failing to extend the flaps past the Flaps 20 position. The 'Alpha' indicator on the flight director came on. I advanced the power to arrest the speed decay; and the First Officer realized the flaps were still at 20. I called for Flaps 30; and then slowed to the proper approach speed. We did not have any indications of stall; or stick shaker activation.This flight was the classic 'chain of events' discussed in aviation incidents/accidents; and looking back; it is easy to see where the chain could have been broken. If I had known that the aircraft's real issues revolved around SEPARATE mechanical issues that would work to compound the problems into a concern of loss of cabin pressurization; I probably would have refused to fly the plane. Additionally; once cabin pressurization became a concern; I could have created additional time for us by telling Atlanta Approach Control that we needed more time; and distance for the approach. Finally; my personal habit is to never run the command speed bug below the flap speed for the CURRENT flaps set. I did so on this approach only because of the multiple distractions. Fortunately; there was no serious consequence to the slow approach speed; and we did not lose cabin pressurization.