The readline "problem" on OS X has nothing to do with ruby
1.8.4/1.8.5/whatever. The issue is that there is a partial "readline"
library in /usr/lib (libedit or something...) and that is what your
build is linking to by default (as /usr/lib comes before /usr/local/lib
in the link path). Rrebuilding using the
--with-readline-dir=/usr/local configuration option should fix it
(assuming you have installed the true readline in /usr/local).