As Qt doesn't work on the iPhone, have you thought about distributing your app as open source for non-iPhone platforms and as closed source for iPhone? Distributing the Qt "port" of your app as open source does not "infect" the iPhone version of your app with the same license (assuming that you wrote all the code yourself, you can decide under which terms you offer each port). That's how the author of Brain Party has done it (proprietary version for the iPhone with an open source port to Linux).

You can of course release your app closed-source for both Qt-based platforms and the iPhone - no problem there

Read that page and your questions shall be answered: http://qt.nokia.com/products/licensing/

(basically, with the LGPL'd version of Qt, you can have closed source code when you link to Qt dynamically IIRC - but IANAL)