When I put my redemption code in, I immediately got three downloads: HD 1080p, SD and iTunes Extras.
Is your device compatible with HD (because if not it will default to the SD version)?
If so, have you selected a preferred HD format in both the Playback and Store tabs in iTunes Preferences?
If you visit the Prometheus product page in the iTunes store, do you have the option to purchase a HD version? If there is, it should say "(Downloading [format])", where [format] is what you have selected in Preferences.
See here for more information.
If your computer ticks all the boxes, I suggest sending Apple a support ticket.
This actually has nothing to do with iTunes. The publisher (Sony, WB, etc..) determines what version you're getting through that digital download. It's also not consistent through releases either. For example, roomate bought the anniversary Blu-Ray release of the original Batman, and the digital download only unlocked the SD version. He purchased it the day it was released. I bought my copy a few days ago, and it unlocked the HD version.
Unfortunately there is nothing you can really do about this, and nothing that Apple / iTunes can do to help you. The publisher decides what you're going to get with that code.