IANAL but my understanding is that paragraph 2 and 4 of the LGPL do apply to your question. Please review the LPGL at http://www.fsf.org/licenses/lgpl.html .
My conclusion is that if you make changes to JBoss itself (and you are not only using it as it is) and you want to distribute the changed code you have to make the source code of the changed version of JBoss (and ONLY of JBoss, NOT of your application which is using JBoss) available at no cost under LGPL license to everyone.
If you sent the changes back to JBoss.org and they are merged into the JBoss source code then JBoss.org will do that for you ;-).
Thanks Tobais. Thats the impression that I got from reading the LGPL. I feel much safer though, confirming it from you folks. Else, there is always the fear that my interpretation may have been wrong.