The short version is, the Pearl River Delta in China, which includes Hong Kong, Macau and other cities MUCH bigger than cities you've heard of - but you probably haven't heard of these, has plans to merge into a mega-city 42 million people (+) strong. The area is known for mining, manufacturing and in the case of Hong Kong and Macau, economic strength and a unique system of capitalist-"communism". I'm aware that they're saying they'll be high-speed rail systems, I guess because of that there shouldn't be any problems with (officially) uniting this entire area right? No urban sprawl issues, congestion/gridlock, environmental issues. I'm sure the current in-between areas are already concrete naturally.
Source -> http://www.boingboing.net/2011/01/28/all-of-pearl-river-d.html