Using iMovie HD what I do is make an iMovie then save it in a format that will play in whatever mode while keeping the original.
If I'm uploading to YouTube I'll go to the File menu, hit 'Share', Quicktime, save for 'Web'. I find that saving in .mov or .mpg works for iTunes.
Use 'Add to Library' in the iTunes File menu to import the movie.
I just did this with a quick movie slide show, it popped up in the Movies panel view in iTunes and played when I clicked on it.
edit: ^Didn't see Cave Man's post; that'll work too.