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).
