There has been some research to that end. The main hurdle there is that MacRuby depends on the Cocoa garbage collector, which is intentionally not supported on iOS. For iOS, they would need to either find another garbage collector that works well with MacRuby and somehow magically Does The Right Thing with all the SDK classes, or else teach the MacRuby compiler to insert retains and autoreleases where needed.
Plausible Labs have released PLBlocks - a version of GCC + a runtime library that allow you to use garbage collection in your iOS apps. Presumably MacRuby could just use the runtime library to provide the GC support it needs.
The BridgeSupport issue (mentioned by another commenter) is more troublesome.
The garbage collector isn't the only issue. Compiled Obj-C code does not retain sufficient metadata (i.e. variadic methods, enums, and such) to do certain "ruby"-ish things. Currently, this is resolved using BridgeSupport, which is also not currently available on iOS...
MacRuby's Clang-based version of BridgeSupport should already be most of the way to solving this problem. (I haven't tested, but I can't see why that would not be the case.)
The Clang-based BridgeSupport preview only improves the metadata generation. There's still no way to package the metadata that is iPhone friendly. Ultimately, a likely solution will be to add the metadata to an extra __TEXT segment and use dlopen/dlsym to fetch it from the MacRuby runtime...
