Symptom : 
Clutter-based applications aren't able to handle mouse clicks. For example, gnome-shell overview isn't rendered when clicking the "Activities" button.

Probable reason :
Clutter scenes are composed of actors throwing events when clicked. The Clutter core uses a kind of multicolor mask where each actor has a different color to pick the currently clicked Actor.
Thus, it reads the color value at the clicked spot and match it with the related actor.  This step is called "picking". An event is then raised to the client code to notify a click on the particular actor.
Current Clutter implementation uses glReadPixels to get the color value (see clutter-main.c:523) :

glReadPixels (x, viewport[3] - y -1, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);

Owen Taylor tends to think that the implementation of glReadPixels in the r600 mesa driver behaves incorrectly and needs fixing.

for me, it worked once (tried last before new years) .. now its very flakey(aka most of the time it works, but sometime it does not) and i get lots of clutter warnings like:

(mutter:26391): Clutter-WARNING **: The required ID of 986895 does not refer to an existing actor; this usually implies that the pick() of an actor is not correctly implemented or that there is an error in the glReadPixels() implementation of the GL driver.

rv770, using mesa master (no gallium, with kms), clutter 1.0.8.

Is this still an issue with mesa 7.10 or git master?

Works here for a while now (rawhide's Clutter and mesa).