If you're not sure, why not start with something like Appcelerator Titanium? It's cross-platform (iOS, Android, preliminary support for Blackberry and even desktop apps) to a very large degree (some UI tweaks may be needed to make it look right) and you can write your code in Javascript -- which is a very useful thing to know even if you don't stick with mobile apps. There are other similar packages (PhoneGap, etc.) but from what I've seen Titanium is much more performant.