AFAIK all rangefinder digitals have APS-sized sensors (between 1.3x and 1.6x crop factor). I think a 20D sans battery grip with an ef-s zoom or a prime is as close as you're going to get without giving up a lot of image quality or spending a LOT of cash for something like the R-D1 or Leica digital.

I'd dearly love a light, portable back up to my 1DMKII.  I'd give up just about everything (ie frame rate, SLR, industrial build etc) except lens/picture quality for something in the 24-70mm and 8 mpx range area.  Anything out there?

Thanks, actually the R-D1 looks interesting.  Maybe in it's next iteration....