With blocks, the semantics and programming paradigms of Objective C are now very close to Smalltalk. Seems like a Smalltalk targeting OS X and iOS would be a very good fit, similar to RubyMotion (which further demonstrates the underlying similarities between Ruby, Smalltalk and Objective C).
I don't think we're that close. If you're looking from Java, yeah, Objective-C looks pretty close to Smalltalk; if you're looking from Smalltalk, Objective-C is a little dot on the horizon. Objective-C is just at its core much more low-level than Smalltalk and considers that a feature rather than a bug.
If you're interested in the idea though, F-Script was actually pretty close to "Smalltalk for Cocoa," though it's more designed as a developer tool for Objective-C programs than an applications development language in its own right. It has an object browser and everything.