Do you have any experience with C#?
Since you have an Objective-C background, it might be easier for you to use Obj-C, and Java for Android/BlackBerry. Otherwise it will be C# for iOS/Android and Java for BlackBerry (you could potentially have to learn 2 languages there).
I have very a strong preference for MonoTouch and find it much more productive than Objective-C, but I came from a C# .Net/Windows background.
I would stay away from "cross-platform" tools such as Airplay SDK, you app will probably be slower and not feel very native in the end. MonoTouch and Mono for Android are more truly native.