Are Cincinnati and Dayton, Ohio, on the road to becoming a single metropolitan area, or "metroplex", not unlike Minneapolis/St. Paul or Dallas/Ft. Worth? The Cincinnati Enquirer has raised that question, with additional commentary from The Urbanophile.
The expectation is that after the 2010 Census, the Cincinnati-Middletown and Dayton Metropolitan Statistical Areas would become a single entity. The rapid growth of the I-75 corridor between the two cities makes such a merger seem inevitable. Perhaps eventually leading to some sort of political merger as a mega-city of three million people!
Currently, the biggest thing connecting the two communities are the two very sorry Cincinnati sports franchises, the Bengals and the Reds. The cities have their own airports, television and radio markets. Their cultural and arts communities rarely interact, with separate professional theaters, symphonies and ballet companies. Hearty workers do commute in both directions. However, as a Daytonian I can say that I do not think of my self as a "Southwest Ohio Cincinnati-Daytonian"... Sometimes Cincinnati does not even feel like Ohio. I often think of Dayton as the last outpost of "the north", while Cincinnati feels to me like a southern city on the wrong side of the river... or as Ms. Cartophiliac often suggests, Cincinnati is its own feudal city-state.
It is going to take more than a Census Statistical merger to pull these two communities together. Dayton and Montgomery County can barely agree to cooperate on combined services. I think it will be some time before we see Cindaynati on the map.