Assuming you have a pointer "main" to your QMainWindow object, you can simply do the following:
main->setWFlags(main->getWFlags() | WStyle_Customize);
This causes the QPEApplication class to ignore the "Cancel" button and pass it on as a normal key event. The only side effect that I've noticed is that the "Ok" button will no longer be mapped to "Return" but if your app needs "Cancel" control, that may actually be a bonus.
This is probably old news to most Qtopia developers, but I wasn't able to find this info anywhere. Sooner or later someone else is going to need this so hopefully this post will save them from trudging through the Qtopia source code.