Also, it's interesting that this would require a distillers license and with that distillery tax rates which are several time higher than for breweries. There is an entire industry of Zima-of-the-year malt beverage products built around avoiding distilled spirit tax rates.
I doubt you'd actually know you were drinking this, since the business would probably keep it on the DL. I'd guess businesses most likely to adopt would be high-volume places where the focus is not beer - like stadiums or casinos.
Even assuming that the concentrate could be mixed to make a good product, a brewery would still have to worry about the quality of the water it is diluted with, not to mention carbonation.
I hear that many of the macro beers are already brewed at higher ABV and diluted for packaging. If this were feasible, I'd think they'd already be shipping kegs of high ABV beer and using an inline mixer to dilute at the bar. And that would be a much better idea from a quality standpoint.