MacOS and Windows both have menu options or hotkeys (WinKey-M for the latter) that let users minimize all windows at once. KDE has something similar, I think, and it would be nice if it were possible to do this in GNOME without minimizing each window by hand.
As GNOME has no 'official' window manager (the project has always taken the stance that any wm that supports the wm hints is 'gnome compliant') this is hard to do in the case of GNOME itself. I know that someone has written such a script for sawfish (perhaps look on the sawfish lisp repository: http://www.sics.se/~lofgren/sawmill/repository.html). You could also pass this onto the maintainer of the wm you use if it is not sawfish. I will keep this in mind as well for our default setup - thanks!
Yes, it would be hard to do this as a global hotkey. However, the gnome-taskbar applet already supports minimizing individual windows, and 'iconify-all' would just be another option on the right-button popup menu over the task buttons. I don't think this would be any more WM-specific than iconifying a single window. It could be as simple as just walking all the buttons in the taskbar and minimizing them one by one.