Just a quick update:
I have made Rational nothrow, fixed toString, and some other minor changes. For the past several months I've used Rational extensively in units (see my other thread), and it works great.
source: https://github.com/Arlen/phobos/blob/std_rational/std/rational.d
docs: http://arlen.github.com/phobos/std_rational.html
What would be the next step, a formal review?
Arlen