In reply to ksjs: Does the temperature and/or humidity of the area the door is hung vary considerably through the year?
If yes, I'd not worry about trying to straighten it out if it works as a door If you need a door to remain just so in those kind of conditions, then either traditional ledged and braced construction or engineered timber doors are the best option, solid wood frame and panel doors will naturally change a little with moisture content as the seasons go round.
If it is going into a stable environment, then applying a significant weight (and possibly some gentle heat) may be able to deform the fibres in the timber back into true, but it's not a sure thing.