This is from Laurent Sansonetti, the original author and long time maintainer of MacRuby. It doesn't say that anywhere until after you buy it, which they should really change since that made me go from "Uh, did some random guys just take MacRuby's code and hack in some extra stuff to sell it?" to "Holy crap so THIS is what he's been working on!"
RubyMotion is based on MacRuby, a widely-used implementation of Ruby created and maintained by Apple for over 4 years.
Starting from a solid and stable code base, the guys who originally created MacRuby now work on RubyMotion. You're in good hands.
Some impressive stuff here, check out the getting started video at around 4:30 The 'positioning a label' live from the simulator command line is way cool. http://www.rubymotion.com/getting-started/