I have already reported the bug (as per your linked thread).
Of note is that I was at the local University Village Apple store here in Seattle last weekend and confirmed this bug on the best 27" iMac (current model) they had in the store using the sample library in AP3.
That machine had the ATI graphics chip and my machine has the nVidia 8800 GT, so I at least verified it is not specific to my hardware.
Dragging images onto the light table displayed the same 'low quality' thumbnail issue I have seen since updating from AP 3.0.2 to 3.1.x.
Scaling the images up seemed to improve the image quality, but this then utilizes too much viewable area of the light table to use for real layout option work (even on the 27" screen at Apple store).
Here is the thread I posted as well in case anyone has interest in the troubleshooting I have already tried:
Thanks for bringing this up again.
Still waiting for someone to actually post that it works correctly for them at all thumbnail sizes in the Light Table function using AP 3.1.x.
I will also send another bug report and include my test at Apple store in the report.