Aaron Hillegass wrote a post on this, and makes a case for learning objective-c.
http://www.bignerdranch.com/blog/ios-developers-need-to-know...
* For those unfamiliar with Aaron, he's one of the foremost authors and teachers of Mac and iOS development -- going all the way back to NeXT. He's gonna be a little biased towards Objective-C, but what he has to say is definitely of interest.