This idea has been floating around for nearly a decade in its current form but it seems to be gathering speed as airlines look to squeeze every penny out of available seat-miles. 

Using the popular Boeing 737-300 as an example, his study calculates that a standing cabin would lead to a 21% increase in passenger capacity while dropping ticket prices by as much as 44%.
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Ryanair tried (and failed) to certify a similar setup and one has to think that they would be the most likely to employ such a seating arrangement.