Edit by EvilSeph from Mojang:
This bug here is actually that an item is dropped when a pickaxe is used to hit a Command Block and has nothing to do with Silk Touch.
I was wondering if you could still pick up mushroom blocks with silk touch, and since it was my test world, I had a command block in my inventory, when it dawned on me. I placed the command block down, went into survival, and instead of the command block breaking and not dropping anything, a command block was dropped.
This can be dangerous, if there is a "rogue command block" amok on a server. If the server has currency, a player could sell this block to a person and get a huge load of cash, meanwhile the player can't do anything with it, for you need to be opped and in creative mode.
Hope this gets noticed!