FULL PRODUCT VERSION :
java version "1.6.0_0"
IcedTea6 1.4 (6b14-1~exp1) Runtime Environment (build 1.6.0_0-b14)
OpenJDK Client VM (build 14.0-b08, mixed mode, sharing)

Please notice though I reported this bug as OpenJDK one, I've also tried:

java version "1.6.0_10"
Java(TM) SE Runtime Environment (build 1.6.0_10-b33)
Java HotSpot(TM) Client VM (build 11.0-b15, mixed mode, sharing)
(called java-6-sun in Debian)

and the latest early access version of JDK 7 from Jan, 29.


ADDITIONAL OS VERSION INFORMATION :
Debian/sid

EXTRA RELEVANT SYSTEM CONFIGURATION :
GTK: 2.14.7
Clearlooks: 2.16.1

A DESCRIPTION OF THE PROBLEM :
GTK-menus don't look like those of native GTK applications.

It seems that they even don't get communicated as being menus, e.g no window manager draws shadows beneath them (neither Metacity with compositing nor Compiz).


REPRODUCIBILITY :
This bug can be reproduced always.