2011-12-02 "Introducing Megapolitanism"
[http://www.thinkurban.org/category/regionalism/]
A recent article from John King at the San Francisco Chronicle mentioned the concept of using the Megalopolitan scale for planning purposes [http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2011/11/25/BA271M30IG.DTL].  The article references the new book by Arthur C. Nelson and Robert E. Lang entitled ‘Megapolitan America: A New Vision for Understanding America’s Metropolitan Geography‘ (APA, 2011) [http://www.amazon.com/Megapolitan-America-Understanding-Metropolitan-Geography/dp/1932364978].
As an example, King mentions the Sierra Pacific Megapolitan Area, seen below as a large geographical area that extends from the San Francisco Bay area all the way into Western Nevada, around Reno.   The region includes 27 counties and includes over 12.4 million people, and its expected to grow substantially in the next 30 years.

As mentioned in the article, the significance of the concept of megapolitan areas is to look more broadly at a larger scale, King, quoting Nelson, mentions that “regions can be more proactive in everything from transportation planning to economic strategies…  to have people look at things a little differently, the whole rather than the parts.” 
While explicitly not a model for mega-regional government, there are some possibilities of what this might mean for regions by looking at larger areas.  As mentioned by King, “It’s too early to say whether the concept of megapolitan areas will catch on as a framework for government policy, much less in terms of how regular people define where they live.”
The significant of megapolitan areas, thus is undetermined.  The overall ambiguity of the defining characteristics of a ‘city’ has led to a lot of questions related to city centers, sprawl, and other hybrid urban agglomerations like edge cities, exurbs, and the shift from urban area to metropolitan statistical areas (MSAs).  This leads to a lot of diversity in definition (outlined in the SF Gate article) -  including the largest megapolitan area (NY-Phil 33.9 million people) to the smallest, fastest growing (Las Vegas 2.4 million).  While Vegas booms, the Steel Corridor of wester PA is creeping along slowly.  In terms of diversity, not surprisingly, the Southern California region has the largest percentage of minorities (62.7%) and the Twin-Cities are the least diverse with 15.5% of minorities.
The terms megaregion, megalopolis, megapolitan area, while similar in nature, are somewhat different historically, spatially, and statistically, so it is worth a look at some of the designations.  A map of megaregions shows the eleven areas in the United States as determined by the Regional Plan Association.

This differs somewhat from a more recent version of Megapolitan areas from a recent essay by Lang and Nelson on Places from Design Observer [http://places.designobserver.com/feature/megapolitan-america/30648/])  They identify 10 megapolitan clusters that exist in 23 megapolitan areas that are similar but slightly different from those above.

The different terms, definitions, and geographical extents makes the concepts a bit difficult to parse, but in general terms, the areas are defined by a population of more than 10 million people that exist within a ‘clustered network of cities’ typically delineated through transportation corridors.
The new interpretation of Megapolitan area builds on earlier concepts to describe a more general ‘transmetropolitan geography’ which is typically thought of more commonly in larger, global areas such as China, Japan, Brazil – which include megaregions of 120 million (Hong Kong, Shenzen-Guangzhou), 60 million (Nagoya-Osaka-Kyoto-Kobe) and 43 million (Rio de Janeiro-Sao Paulo).  While the concepts are similar, the scale of these new global areas are immense in comparison to the US.
Interestingly enough, the term has been used since the 1820s, and the conceptual usage of the concept of Megalopolis as a grouping of urban areas within a region dates back almost 100 years.  This includes references by Oswald Spengler in The Decline of the West (1918) and Lewis Mumford in The Culture of Cities (1938).  The most popularized recent usage was from 1950s and 60s, in the book on the Northeast United States by Jean Gottmann entitled ‘Megalopolis’ (1961).