There's a guitarist I read about in a music mag ('Guitarist', IIRC) who defines what he plays as variously 'heavy mental' and 'acoustic thrash'. Not heard him myself (only having a small CD budget!) so I wouldn't know what it's actually like. This thread just kind of triggered the memory. Does this ring a bell with anyone?

And of course there's always Under The Bridge (RHCP). A couple of guitars can do a pretty heavy acoustic version of that.