One of the problems I'd have with a certification of this sort is that
it focuses only on the least important aspects of the language -
syntax, nomenclature, conventions, etc.

What's really important is good design skills, especially using Ruby's
tools (OO, metaprogramming, agility, etc.).  And these are the type of
things which are almost impossible to test on a curicullum based,
multiple choice, certification.

Knowing a language means little more than knowing a bunch of syntax.
It's a world apart from knowing how to develop.  Right now, there are
only two languages where it's usually safe to assume that someone who
knows "language" is also a good developer (Ruby and Python) - I'd hate
to see Ruby fall from that.