Just Heroic Mode, however the drop rate I have no idea.
There's no rough estimates on WoWhead and there really hasn't been all that much talk about them by Beta players (I guess because the items didn't matter since your gear just scaled up/down for everything).
Their Ilvl isn't high enough to warrant some insanely low chance like 2-5% .. But it might be within the 10-20% Range.