Why sometimes does my lower menu (ADWLauncher - Dockbar) disappears?
I had same problem before.
user:eldarerathis was correct. It is swipe gesture that suppose to hide menu when you don't need it, but AFAIK, there's no way to turn it off. 
So, to get it back you should do gesture starting roughly a bit above Browser icon and swipe down(or up).

I've fixed that another way - simply installed different launcher. Problem didn't appear