MS have a transition guide for moving off of their VM:
http://www.microsoft.com/mscopr/java
Sun also has migration guides in the JDK docs:
http://java.sun.com/j2se/1.4.2/docs/guide/deployment/deployment-guide/upgrade-guide
And there is a specific forum for discussing this:
http://forum.java.sun.com/forum.jspa?forumID=481