You'd need to familiarise yourself with Xcode (Apple's development platform), and learn Objective C.
I speak Java fluently, but had quite a lot of trouble getting accustomed with ObjC (to the point that I gave up). But you've got a different background (and experience) than me.
An excellent resource for ObjC is a series of lectures from Stanford, focused on iPhone dev (
http://itunes.stanford.edu/)
Good luck:)