My worry is that it would take off so well that the ruby gem community could get fragmented into libraries for RubyMotion and libraries for regular ruby projects.
There are already exclusive libraries for MRI/JRuby, so adding a third platform doesn't seem like it would break the world. Tools like Bundler do a great job of keeping things moving smoothly.
I'm supposing that they will probably implement it as another platform option like you can do right now for JRuby/Windows/etc. It shouldn't affect much. I haven't looked at the documentation, though, so I can't say for sure.
My worry is that it would take off so well that the ruby gem community could get fragmented into libraries for RubyMotion and libraries for regular ruby projects.