UIManager.getIcon(iconKey) does not return tree icons in GTK LAF, thus breaking the UIManager public contract for getIcon() which states specifically that it will return an icon if given an icon key.
Custom tree like components therefore cannot reuse these icons and will lose fidelity with the current LAF.

This affects systems with GTK LAF, such as Linux. This has been verified in JDK1.6 and JDK1.7.

Compile and run the attached program on Linux, and you'll see how the icon is painted in Metal LAF but not in GTK LAF.