That key's already used for Emacs! (It's "set mark"). I use it, anyway :-)

But seriously, wouldn't a status bar applet be better UI than an easy to accidentally type keyboard layout switch? If you do switch by accident and you don't know what happened, you could really do with a visible way to see what the state is and to change it back (because you might not know the key sequence). control+space is not hard to mistype when you're typing other control+letter combinations a lot.