In OS X the keyboard shortcut Command + H (Cmd-H) hides the current app windows.
Mac OS X keyboard shortcuts: "Command-H Hide the windows of the currently running application"
This is a very old behavior. It made sense in Multifinder days; it's worse than useless in Snow Leopard -- it is inconsistent with Expose and Spaces. In Lion it's not quite so bad, but my fingers long ago mapped Cmd-H to "show history" in all browers.
I'm always hiding Safari, and having to unhide.
Today I realized I could do something about this. In keyboard shortcuts I mapped Cmd-H to Help, a system wide command. That's still a problem, but at least I don't hide my windows when I try to access history.
Next I'll try to remap the Safari shortcut for history to cmd-H, but even this is a big help.
Update: If I map Cmd-H to a system action like help file, it trumps mapping it to an app-specific menu item like Safari history. I decided to go with cmd-H for Safari, but outside of Safari it doesn't currently do anything. I might be able to use it for other app-specific menu actions. This will help!