I’ve already mentioned that the upcoming Nimbus look-and-feel is not being hosted in a usual location (javax.swing package). While at first the NimbusLookAndFeel class was in a sun.swing.plaf.nimbus package, it has now been moved to a new home – com.sun.java.swing.plaf.nimbus, whose parent package also hosts GTK, Motif and Windows look-and-feels.

Bug 6616742 is the reason – Nimbus was not available in unsigned applets due to class loading restrictions on internal sun.swing packages. It also fixes 6618285 – using the NimbusLookAndFeel class directly in the source code.