Is this a known bug?
Yes, known issue...will be fixed. In the meantime the workaround is to process the images as sRGB (gamma 2.2) or ProPhoto RGB (gamma 1.8) and convert to grayscale in Photoshop.
Thanks - guess it's back to CS5 then. 8)
There is still no solution?
It'll take until ACR 7.4 is released to fix...in the meantime, use the work around.