@PHeMoX:

You're using shaders along with the blur effect, right ?

I was able to make the code work on my testbed code here, but when I merged it with our latest tarball (wich includes all shaders/effects we intend to ship in the final version) the motion-blur started to act exactly like yours. I has nothing to do with the code 'per se' but an incompatibility with some shader. I'll try to disable them one-by-one to see if I can isolate the culprit.