Show hidden files in Finder but not on desktop with Mac OS X
Maybe not exactly what you're looking for, but useful to know: in Save/Open dialog boxes in applications, you can press Command-Shift-. to show/hide hidden files.
As far as I know, no: the desktop is Finder.
But there is a reasonable workaround: you make a tiny AppleScript app that toggles AppleShowAllFiles for you and drag it to your Finder toolbar, making a button to run it. See this article at Macworld.