Wow, I had an interesting time trying to get my eclipse plugin, developed in linux using Java 1.6 to work on my OS/X JVM which is a 64 bit JVM. I was getting all sort of problem to do with the SWT libraries java.lang.UnsatisfiedLinkError: Cannot load 32-bit SWT libraries on 64-bit JVM. I found a few work-arounds for this using google, but seemed very complicated and as I am a mac newbie - i thought i better not.
So I decided to change the plugin to 1.5 - so I set the runtime environment to use a 1.5 version JVM, but then I was gettng problems with the version number of the compiled classes"bad version number in .class file". So I changed the build environment to a 1.5 JVM. I thought this would fix it - but no. So then i done a search and found a way to rebuild the project (project -> clean..). But still no joy. So then I removed the classes files manually - thinking the newly build files with the JVM 1.5 werent getting written because the 1.6 version were still there, but no. So after about an hour I figured out what was going wrong - there was still something in my project set to 1.6 - the java compiler settings for the project!!!?!