Machine learning libraries for Ruby
I'll go ahead and post an answer for now; if someone has something better I'll accept theirs. 

At this point the most powerful option appears to be accessing WEKA using jRuby. We spent yesterday scouring the 'net, and this combination was even used by a talk at RailsConf 2012, so I would guess if there were a comparable pure ruby package, they would have used it.

Note that if you know exactly what you need, there are plenty of individual libraries that either wrap standalone packages like libsvm or re-implement some individual algorithms like Naive Bayes in pure Ruby and will spare you from using jRuby.

But for a general-purpose library, WEKA and jRuby seem to be the best bet at this time.