When you create a new project you can specify the source level and the JDK to use with it.
This is an important fact because depending on the source level the GUI builder matisse will use new classes available in JDK6 or not.
Before JDK6 or using 1.5 source level, swing-layout project is used to provide some useful swing layouts.
As you can read in the project home page: As with other swing-labs projects this is exploratory work, parts of this that are successful will be pulled into future JDKs. The baseline work has already been pulled into 1.6.
This means that for source level 1.6, NetBeans builds the GUIs using the same (or similar) classes located in the JDK6 instead of swing-layout library.
Remember it, if you want to create GUIs that must run in older version of JDK and not only in JDK6.