With the latest release of Mesa taskbar became white (not transparent) using kwin GLES. With usual kwin it is transparent.

OS: Kubuntu 13.10 x86_64
Kernel: 3.12RC3
Graphics: Radeon HD 2600 XT

kwin_gles --replace & 
[1] 2533
eugene@desktop:~$ QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 14: reading configurations from ~/.fonts.conf is deprecated.
QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread
QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread
eugene@desktop:~$ OpenGL vendor string:                   X.Org
OpenGL renderer string:                 Gallium 0.4 on AMD RV630
OpenGL version string:                  OpenGL ES 3.0 Mesa 9.2.0
OpenGL shading language version string: OpenGL ES GLSL ES 3.0
Driver:                                 R600G
GPU class:                              R600
OpenGL version:                         3.0
GLSL version:                           3.0
Mesa version:                           9.2
X server version:                       1.14.3
Linux kernel version:                   3.12
Direct rendering:                       yes
Requires strict binding:                no
GLSL shaders:                           yes
Texture NPOT support:                   yes
Virtual Machine:                        no
kwin(2533) KWin::EglOnXBackend::init: Cannot enable v-sync as max. swap interval is 0 
QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread
QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread
kwin(2533): Failed to compile fragment shader: 
"0:2(14): error: no precision specified this scope for type `vec4'
0:4(13): error: no precision specified this scope for type `float'
0:5(13): error: no precision specified this scope for type `float'
0:6(13): error: no precision specified this scope for type `float'
0:7(13): error: no precision specified this scope for type `float'
0:8(13): error: no precision specified this scope for type `float'
0:9(13): error: no precision specified this scope for type `float'
0:10(13): error: no precision specified this scope for type `float'
0:14(7): error: no precision specified this scope for type `vec4'

kwin(2533): Failed to link shader: 
"error: program lacks a fragment shader

On bugs.freedesktop.org says that bug is in kwin's shaders. Please see their comment here: https://bugs.freedesktop.org/show_bug.cgi?id=70169#c2

Reproducible: Always