Open Source SWING Native Look And Feels
There is no danger in open sourcing the native look and feels. The various platforms look and behave a particular way so there are no potential forking decisions to be made in those packages. Let the community catch and fix inconsistencies with the native platforms for faster turnaround times with less Sun resources being devoted to it.
I think all you need to do is to come up with a licensing policy that allows you to redistribute the open source libraries with the Java runtime without it also becoming open source. And I believe even the existing LGPL would allow for that.