The iPhone uses Cocoa, Java dominates on other mobile devices, there’s a lot of interest in Erlang for highly concurrent systems, if your market is web you need Javascript (unless you use GWT, Pyjamas, or soon-to-be-released Red), Lua is starting to dominate in game scripting, C# is king for Xbox XNA games, and some day Nintendo will realize that requiring a separate, secure office and charging $2,000 - $10,000 is driving indie game developers to the iPhone/Touch.