So I finally got an answer from devrel here: The ATI driver assumes that all rendering is done in linear color space in case AA is enabled. Something one could argue on (e.g. linear color space is only needed during the _downsampling_ process).
And therefore the ATI driver just completely ignores the SRGBWRITEENABLE flag in the AA case.
At least for a clear operation it would be possible to do the color conversion in the driver before issuing it on the GPU. We only have 8 bits per color for the clear operation, so depending on the value sRGB or linear based encoding could be beneficial.
Summa summarum: Please either fix the driver or return an appropriate error code.
What's the graphic card you are using? Radeon HD 5000 series or something else?