I just bought a Panasonic GH1 (micro 4/3) kit with the 14-140 lens as a possible smaller and lighter alternative to my Nikon D300 with the Nikon 18-200 VR lens (similar zoom range after accounting for sensor size), and took both to Yosemite over last weekend. I took a few of the same photos with both so I could compare them, and thought I'd report on what I found here...
I processed both the images below with ACR (starting with raw), using a similar approach for both. Despite the difference in sensor size, I thought the GH1/14-140 produced very nearly the image quality of the D300/18-200. The D300 was very slightly sharper, mostly in the corners, but that's the only quality difference I could find, and the difference was quite small. (Differences in contrast & brightness are likely just the variation in my post-processing; I processed each completely independently, without comparing with the other one.)
I still don't feel very comfortable with using the GH1 and its features yet, so I used the D300 for most of the trip, but that will get better with practice.
If you'd like a smaller, lighter alternative to a DSLR, the GH1 (or GF1) route is something to consider.
(I'm also posting this to the micro 4/3 forum.)