system ~/projects/jruby $ jruby -e "p 'FØØ'.downcase"
system ~/projects/jruby $ jruby -e "p 'føø'.upcase"
A crappy workaround for now would be str.to_java.to_upper_case.
Fortunately, UnicodeUtils gem works in JRuby, but dragging a whole gem for such miniscule task seems inappropriate.
So UnicodeUtils.downcase(str) looks a bit like crap, but it's a workaround nontheless.
Actually it appears that MRI does not case fold these characters either, so I'm closing this.
See also rubinius/rubinius#2053 (comment)