iTunes: how can display multiple versions of a movie in SD and 1080p/720p HD as one movie?
Download Subler (http://code.google.com/p/subler/). It open your HD version of the movie in Subler and make sure you check off and mark it as an HD version. This then adds the meta-data so it know's it an HD movie.

Also, make sure it's marked as a Movie. Normally it'll appear as Normal.

Finally, you need to add a meta-tag on the first tab. ContentID needs to be populated, and it has to be unique to your library, and the same for both files. Basically, pick a random number and shove it in.

You can then re-add the movies and it'll bundle them together. At this point you can remove them from iTunes (select and delete). I would choose the Keep Files option, and just drag them back to iTunes to get it re-add them.

Personally, I use iVi (available from the Mac App Store) and if you select the options for both SD and HD output it will do all the work for you.