I'm still compiling a list of the annoyances I encounter when converting from Java to Objective-C, which I will publish soon. But so far the worst one was to be careful when using the math functions like abs instead of fabs! The abs function works with ints and if you try using it with a float, you are going to get some bogus result. Why why aren't such methods overloaded!?!
Wasted a good two hours trying to hunt down such a bug in some code which worked fine in actionscript.
Mental note: remember as well sinf, cosf, asinf, acosf
Migrating current actionscript code to Xcode... check :D