DxO Mark has just reviewed the sensor in the new Nikon D5100. It's the same 16MP sensor from Sony that's used in the Pentax K-5, a camera that I enjoyed enormously, and the D7000. Aside from some goodies like a large buffer, high-speed shooting, and some more pro-oriented controls, the picture quality of the D5100 looks like it would be identical to the D7000. Which is to say, excellent. DPReview did a test between the D7000 and Canon 60D which illustrated the low noise floor of the new sensor. Spoiler: The Nikon D7000 hands the EOS 60D its ass on a platter. These new sensors are on a different plane than the competition.
It makes me all the angrier that Olympus had the gall to release the E-5 with a three-year-old sensor and try to sell it for full price. If the Olympus E-3 hadn't been such crap, no one would even be interested in the E-5. The only thing Olympus had going for it, and truly likely the only reason that they sold any cameras at all, is its lens selection. Olympus' semi-pro and pro lenses are among the absolute best on the market. And focal-length to focal-length, the Oly lenses are half the weight (or less) of larger-sensored cameras.
I'm not changing my opinion that your best bet would be 4/3's and then a full-frame camera if you want the best quality, but with cameras like the Pentax K-5 being so small, that's primarily because of the lenses. I once wrote that Canon's creation of the 7D might indicate a newfound focus on the enthusiast cameras, and that a collection of new, high-quality EF-S lenses was on the way. That was wishful thinking. It's obvious that Canon would rather try and protect their cash cow than actually blaze a trail into any new markets.
So we are stuck with excellent APS-C cameras that have crap for lenses. One of hte advantages of a smaller sensor is, with lenses DESIGNED FOR THE SENSOR, you get a broader depth of field for any particular aperture. That allows more things to be in focus. But you only get that benefit with lenses designed for the 1.6x crop factor. Simply slapping an ordinary EF lens onto the back of the 7D nets you no benefits except for slightly better vignetting, and in return provides lower resolution. Yay.
Sony has their NEX Cameras, which might finally result in good, APS-C glass. As might the rumored Fuji mirrorless system camera coming next year. Until then, m4/3's is the only system that's pushing the boundary of general photography, and for that, it gets my money.