Welcome to the wonderful world of hardware acceleration.
The bottom line here is that the use of the GPU inside of Premiere Pro isn't quite perfected yet. Ideally the only difference between using it or not would be the speed of rendering, with absolutely no difference in the content itself. But that isn't quite the reality yet.
Perhaps CS6 will use OpenCL (so we can also use ATI cards) and will get this one right.
It seems strange to me why I am seeing poorer quality exports with software rendering. As I mentioned and as shown in the attachment just exporting a still using software rendering gives a much softer jpg then with having GPU assisted rendering. Does it mean those who do not have GPU rendering are losing out on quality? I somehow doubt it but that is what I am seeing.
Well I thought I would bump this post one last time. Does anyone see better export quality with GPU assisted rendering "on" compared to software rendering as per the stills in the link in my first post?
Sometimes I need to use software rendering and I am seeing poorer quailty in both video and exported stills.