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This is a dupe of Can you use MacRuby to develop applications for the Mac App Store? (more or less).
Is it possible to build an iPhone app using MacRuby alone, without any knowledge of Objective-C?
In short, no.
All of the APIs (system frameworks), documentation, dev tools, examples, and the majority of the development community use Objective-C. You are going to have to know Objective-C through and through to be able to write an app against the iOS or Mac OS X system APIs anyway.
As well, the MacRuby runtime uses the Objective-C garbage collector that ships with Mac OS X. While porting that collector to iOS/ARM is likely not that hard (the source is available), the system frameworks don't support it; it won't work.