So far the closest I've found is HTC Droid Incredible: thin, no keyboard, responsive touchscreen, quality camera (don't have an iPhone 4 so can't compare), accurate GPS (saved my ass in a ghetto) and easily rootable (maybe? I've heard its not hard, but haven't felt the need to).
The Samsung Galaxy S would be exactly that phone if it weren't for the bugs that make it lag badly. Root it and install Froyo, though, and it is a beauty.
I don't know how true the rumours of a Samsung-made Nexus are, but I wouldn't be surprised given the quality of the Galaxy's hardware.