I'm a long-time pro who uses the big cameras at work, but for personal work and hiking I've been using the Panasonic m4/3 system. Love it. I use a GF1 with the 14mm and 20mm lenses, and my wife now has a G-1 with the 14-45 and 45-200. Image quality is very good even at higher ISO values, shooting raw files and processing in the latest version of Lightroom (BIG differences in noise handling in the Process 2010 version.)
I've been using the GF1 and the 20/1.7 as an "everyday carry" camera at work, and the images from it have been quite good. Good enough for print publication and/or very good 13x19 inch prints. Not only are the small cameras easier on my back and neck, but they tend to avoid scaring the subjects too. Carrying a small, quiet, unobtrusive camera with very good image quality is a joy. I'll be buying another m4/3 camera and trying to integrate them more into my professional work, and of course I'll keep taking them on the trail.