Good news! With the new iTunes 10.6 update, it looks like 1080p pony downloads are fully available. All you need to do is update your program, and hit edit-->Preferences-->Store, then select 1080p instead of 720.
The bad news? iTunes doesn't downscale it for lower resolution monitors.
Best program ever.... You will need to download the lower 720/480 if you don't have a monitor that supports full HD, (my laptop that is connected to my 1080 TV for example). I'm sure there are workarounds out there, but the default iTunes setup straight up refuses to play them.