All of the rendering options in LR 3 beta 2 are available in Camera Raw 6.0. The switch you're looking for is the one you mentioned, i.e., Process Version. The default value is 2010, which will enable the new sharpening and denoising methods developed for Camera Raw 6 and Lightroom 3.
Why when I was in LR Beta 3 and I toggled back and forth between the two rendering settings, there was a very noticeable difference in the detail (and no major luminosity difference) when zoomed up at 100%, and then in the new ACR 6.0 when I toggle back and forth between the 2010 and the 2003, I do not see much detail change, but I do see substantial luminosity changes?
It would be great if you could provide an example original raw file. I cannot answer your specific question unless we have an example to work with.
I will try to do that this weekend, thanks.