I'm fairly certain that iOS 5 was stated to be free, and even if it wasn't explicitly stated all iOS updates have been free for iPhone users since iPhone OS 2, and for iPod touch devices for the past update or two, so there isn't any reason why that should change.
The only customers who may have to pay to upgrade are iPad 1 owners. I believe I remember hearing (or seeing?) in the terms that the iPad would receive one major iOS update for free (iOS 4.2.1). I don't think it specifically states that the second major update would be paid only, but it's a possibility.
I don't think ios upgrades will be paid ever again. They are just going to eventually cut off the old hardware anyway. Also the whole support structure (genious, SHSH, phone replacements) becomes easier as they don't support old ios versions for the people that don't want to pay. They likely actually save money on the back end. Everyone wins.
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I was under the impression that all updates were free as long as your hardware was upgradable?