I understand your concerns here. Fortunately, the features users receive are permanent (for a given install) and we allow free-form responses (some guy actually wrote "Tweeting for upgrades sucks" in order to get his upgrade - and we're totally cool with that).
Mostly importantly, it's been a really interesting experience in metrics, marketing, user feedback etc. Not an especially profitable one, but valuable in our understanding of how this sort of thing can work.
I think what's against the app store terms is to show disabled features that are only available in the paid version of an app. (which Apple has done for years in QuickTime ironically)
This probably falls in a grey area: if you compare it to game levels that are only activated after you achieve something, it's pretty similar in spirit. Now that I think about it, I think some games are borderline then…
I could have sworn that was against app store terms. If not, it still feels kinda slimy.