If Qt becomes a major player for developing iPhone apps, that gives more choice to everyone if the apps are compiled for all the supported platforms (ie. Maemo and Symbian). The developer has a wider available audience for his app, for paid apps will mean more potential profit.

Upside from a user's point of view is they aren't stuck with one platform and can choose the one they like, eg. N900, N97 or iPhone etc rather than pick the phone due to the number of apps available.