Also check out this post: http://surdilovic.wordpress.com/2012/08/06/process-and-task-form-editing-in-jbpm-designer/
This form editing will be available in the next Designer release, but you can already start using it by building designer from source:
git clone https://github.com/droolsjbpm/jbpm-designer.git
cd jbpm-designer
mvn clean install
at this point go to the target directory and replace your current designer.war with jbpm-designer-2.4.0-SNAPSHOT-jboss7.war for example if you are running on JBoss AS7.
Don't forget to clear your browser cache before starting to use a new version :)
Hope this helps.