Seems the click-and-hold is only a Dock thing. I can't get the left mouse button to bring up a contextual menu with either my trackpad or Mighty Mouse in, say Safari or the desktop. Anything that's in the Dock, however, will bring up the context menu a-la control/right click with a click-and-hold.
To get right-clicking on the touchpad, I had to go System Preferences > Keyboard & Mouse > Trackpad, then check "Place two fingers on the trackpad and click button for secondary click". Or, check the "Clicking" option and check the "Tap trackpad using two fingers for secondary click".
To get right-clicking on the mighty mouse I had to go System Pref > Keyboard & Mouse > Mouse and change the right side to "Secondary Button".
I'm seeing the same thing you do, that a long click only works in the dock. I'm not seeing anything in System Preferences saying anything about a long click or a click and hold.
OS X really seems to not want you to right-click, but even though I try to use the OS with its default settings, I really want the right-click.