Objective-C without Cocoa is not so useful and won't bring you much closer to haveing a working iPhone codebase. You'd probably be better off writing your core in C with Core Foundation and using either Java or Objective-C for the platform specific parts. Apple has open sourced a large chunk of Core Foundation as CF-Lite, and it's toll-free bridged with Cocoa on OS X (i.e. you can use many CF classes interchangeably with their Cocoa counterparts).
