Ruby to JRuby guide

Is there something available for developers that will show you how to
migrate your rails app to JRuby. I'm referring to stuff like "If you
use ImageMagick to resize your images, here's how to do it with a java
component" or "If you use ruby-net-ldap, here's how you do it in java"

You should probably look at Ola Bini's JRuby on Rails book. Its out
maybe a week now? Timely question.

RMagick is certainly the tricky one. But if you follow the JRuby wiki,
you will notice that they have a plan to port a lot of the most
popular MRI C-Extensions to JRuby (or at least get a lot of community
contribution to achieve the same effect).