I can confirm that, running matlab on current Hardy, with libxcb1 (1.1-1ubuntu1) gives the following :
andrzej@andrzej-laptop:~$ /opt/matlab/bin/matlab
Locking assertion failure.  Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0xb5aee767]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_unlock+0x31) [0xb5aee8b1]
#2 /usr/lib/libX11.so.6(_XReply+0xfd) [0xb5e5a1bd]
#3 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff828ce]
#4 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff5f067]
#5 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff5f318]
#6 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so(Java_sun_awt_X11GraphicsEnvironment_initDisplay+0x2f) [0xaff5f61f]
#7 [0xb0cd7ecd]
#8 [0xb0cd0edd]
#9 [0xb0cd0edd]
#10 [0xb0cce249]
#11 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c40d]
#12 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x6310378]
#13 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c2a0]
#14 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so(JVM_DoPrivileged+0x363) [0x6272153]
#15 /usr/lib/jvm/java-6-sun/jre//lib/i386/libjava.so(Java_java_security_AccessController_doPrivileged__Ljava_security_PrivilegedAction_2+0x3d) [0xb2d3896d]
#16 [0xb0cd7ecd]
#17 [0xb0cd0d77]
#18 [0xb0cce249]
#19 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c40d]
Locking assertion failure.  Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0xb5aee767]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_lock+0x2e) [0xb5aee81e]
#2 /usr/lib/libX11.so.6 [0xb5e59518]
#3 /usr/lib/libX11.so.6(XGetVisualInfo+0x26) [0xb5e500a6]
#4 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff5e319]
#5 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff5e565]
#6 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xaff5f3c9]
#7 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so(Java_sun_awt_X11GraphicsEnvironment_initDisplay+0x2f) [0xaff5f61f]
#8 [0xb0cd7ecd]
#9 [0xb0cd0edd]
#10 [0xb0cd0edd]
#11 [0xb0cce249]
#12 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c40d]
#13 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x6310378]
#14 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c2a0]
#15 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so(JVM_DoPrivileged+0x363) [0x6272153]
#16 /usr/lib/jvm/java-6-sun/jre//lib/i386/libjava.so(Java_java_security_AccessController_doPrivileged__Ljava_security_PrivilegedAction_2+0x3d) [0xb2d3896d]
#17 [0xb0cd7ecd]
#18 [0xb0cd0d77]
#19 [0xb0cce249]

Like exactt wrote - exporting LIBXCB_ALLOW_SLOPPY_LOCK=1 doesn't change anything, probably because it's already on by default.
exporting LIBXCB_DISABLE_SLOPPY_LOCK=1 brings up the "old" error :

 /opt/matlab/bin/matlab
Locking assertion failure.  Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0xb5a70767]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_unlock+0x31) [0xb5a708b1]
#2 /usr/lib/libX11.so.6(_XReply+0xfd) [0xb5ddc1bd]
#3 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xb015f8ce]
#4 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xb013c067]
#5 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so [0xb013c318]
#6 /usr/lib/jvm/java-6-sun-1.6.0.05/jre/lib/i386/xawt/libmawt.so(Java_sun_awt_X11GraphicsEnvironment_initDisplay+0x2f) [0xb013c61f]
#7 [0xb0c59ecd]
#8 [0xb0c52edd]
#9 [0xb0c52edd]
#10 [0xb0c50249]
#11 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c40d]
#12 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x6310378]
#13 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c2a0]
#14 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so(JVM_DoPrivileged+0x363) [0x6272153]
#15 /usr/lib/jvm/java-6-sun/jre//lib/i386/libjava.so(Java_java_security_AccessController_doPrivileged__Ljava_security_PrivilegedAction_2+0x3d) [0xb2cba96d]
#16 [0xb0c59ecd]
#17 [0xb0c52d77]
#18 [0xb0c50249]
#19 /usr/lib/jvm/java-6-sun/jre//lib/i386/client/libjvm.so [0x621c40d]
MATLAB: xcb_xlib.c:82: xcb_xlib_unlock: Assertion `c->xlib.lock' failed.
Aborted (core dumped)