A CRJ200 departed an airport on the first flight of the day with an ambient temperature of -15F. After takeoff a PASSENGER DOOR EICAS message directed that they land at the nearest suitable airport. While returning to the departure airport a NOSE WHEEL STEERING message alerted when the landing gear was lowered.

Narrative

I was the pilot flying. The clearance altitude on takeoff was 10;000 FT. Soon after takeoff we received a caution MSG. PAX DOOR HANDLE. Within three seconds of that message we received a Master Warning PASSENGER DOOR message. The pilot not flying ran the QRH procedures as I requested a level off at 8;000 FT. The QRH requires 'land at nearest suitable airport'. So we planned a return back to the departure airport; advised the passengers of the situation. Although very cold at the departure airport (-15 degrees Fahrenheit); the runways were clear and the taxiways were mostly clear. With the help of the pilot not flying we completed all the landing tasks and planned for a visual approach. On base; I requested for the gear to come down. As the pilot not flying put the gear down; we received a Caution MSG. NOSEWHEEL STEERING. I decided to continue the approach and planned to get off the runway using differential braking as necessary. Once off the runway and on the taxiway the aircraft became too difficult to maneuver safely so I stopped and requested a tug to the gate. Advised the passengers of the tow in and went to gate after about a 12 to 15 minutes wait for a tug. I believe this may have happened because of the outside air temp causing difficulties in the pressurization system to function properly. As to why the nosewheel steering gave out; not sure.