First of all, there are 2 model numbers listed, the PRS-T3 and the PRS-T3S. The S could stand for a touchscreen, but that's not completely clear. But one detail I can tell you is that these models were tested for Wifi (though one report mentions Bluetooth).
I can also tell you that the devices have the same set of buttons as the previous models and measure about 4.22" by 6.22", giving them a 7.5" diagonal measurement. Based on the photos and the diagram discovered in one of the test reports, my current estimate places the screen at 6" (and not the 6.8" screen found on the Kobo Aura HD).
And as you can see from the images above and below , the new T3 looks nothing like Sony's current ereader, the PRS-T2. The T3 has a more traditional rectangular shape and it looks much more like a tablet.
Sony has not yet shared any details on the release date, price, or specs,but if they follow their previous release pattern this ereader will probably launch in the US and Europe in August 2013.
Update: I've just learned that someone added a couple revealing photos to one of the test reports. This post has been updated with the new photos and info. Thanks Nathan!