well, i use final cut pro and not imovie so i am not sure how to do it in imovie.
for final cut pro, i go to file>export>using quicktime conversion
then i select convert to quicktime>options
from there, i select the ntsc dv format and select best settings, then progressive.
However, i am not quite familiar with imovie so hopefully, somebody else can help you.
I also found it helpful to capture in progressive. to do this make sure both the CAMERA AND THE MAC is set up to a progressive codec. in some cases, progressive settings may be under field dominance in three choices of top, bottom, or none. by selecting none, it will be a progressive mode.