Cannot speak of the 320, but on the 330 and 380 -
Before take-off, all outflow valves open, to ensure there is no differential pressure between the air inside the aircraft and the external air.
Take-off Phase, when all doors are closed, the oleos are compressed and (at least 1/(3) engine/s) at take-off power, the outflow valves close, There is no 'pre-pressurisation' with the packs Off. After 'Lift Off' the cabin rate of climb is determined by the cabin leak rate. (200fpm on the 380)
After thrust reduction and when the 1st pack is turned on, the outflow valves will open to accomplish the programmed cabin pressure control.
This stuff should be known before you ever get to strap an aircraft to your bum.
edit: Field in Sight, you beat me to it as I typed, though I would change your "sea level air pressure" to ambient air pressure.
Last edited by Trent 972; 19th September 2010 at 12:59.