- It could be bound to:
M-#
M-_
M-+
M-SPC
C-x SPC
C-x r C-SPC
C-SPC C-SPC C-SPC
C-x C-x C-x C-x C-x C-x
Just a small comment from someone using a Swedish keyboard layout: many of the key bindings suggested in this thread are really awkward to type and I would guess some of the other European keyboard layoyts are causing the same problems. For me, any combination that uses a modifier key (C- or M-) together with a key that I need to use AltGr to type, are tough ones. For me, this includes all bindings that has keys like @, ^, \, [ and ] in them, just to take some examples. Yes, I sometimes switch to an English keyboard layout to make use of the "classic" bindings, but I don't think we should expect that everyone does this. And yes, I do know about input methods...
I would personally prefer this to be placed somewhere under the C-x r keymap (I like C-x r C-SPC, even if it removes one of the bindings for point-to-register), for consistency and discoverability, as some here has already stated.