I am looking at my guide now and many are gone. Some are still there. First noted that about 2 days ago.
This is due to a new firmware upgrade Dish Network released. You can go into the menu to get the SD versions of these channels back into the guide.
yeah Dish switched that so if there is a HD channel that you sub to the SD one is hidden. Directv does the same thing which is so nice...doesnt clutter up the guide