There appear to be several changes in the autohide feature in 12.04, at least as it works on my desktop system which uses a 1920x1080 monitor in portrait mode. I am using the 3D Unity implementation, with a low end ATI graphics card.
1) In a fresh install, autohide is turned off. New Ubuntu users will probably not even realise that the autohide function exists, unless they are shown by someone, or stumble across the setting in the Appearance panel.
2) With autohide on, when no programs are running, the launcher is hidden. This is a change from previous versions, which showed the launcher when nothing was running.
3) My experience about how autohide works seems to be different than that of a couple of other posters. On my system, autohide is responsive not to how far you move the cursor, but to how fast the cursor is moving when it is beyond the left edge of the screen. A slow long movement a long way past the screen edge creates a gray flicker at the screen edge as if the launcher is just on the verge of showing itself. It takes rapid movement to make the launcher show. In practice, I need to jerk or snap the cursor past the left edge of the screen to reveal the launcher.
4) The range of sensitivity available for autohide isn't big enough. Even turned up to high, I have trouble generating enough cursor velocity to get the launcher to activate.