iOS apps cannot have app icons with an alpha channel. They must be 24-bit only. The iOS build must detect if the image is 24 or 32-bit.
If it's 32-bit, then it will need to either A) show a warning and automatically convert the image to a 24-bit image or B) fail the build. We may choose to default to one of these options and allow a tiapp.xml property to change behavior. Which option we go with will depend on input from Chris Barber, Ingo Muschenetz, Eric Merriman , Fokke Zandbergen, and Chee Kiat Ng.