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The ACR 5.6 RC supports new cameras and enhances the support that ACR 5.6 had as Beta for the Canon 7D, G11, and S90 as well as some uneven-green-channel maize-pattern fixes on the Mac.
The splash-screen or Help / About for LR 3b says it has the ACR 6.0-compatable RAW engine. Major version numbers of ACR and LR and CS are synchronized. Since ACR 5.6 is still ACR 5 it matches CS4 and LR 2.x.
As far as seeing the enhancements in the LR3 rendering, this is governed by the so called "Process Version" associated with each image.
Process 1 is the old LR 2.x/ACR 5.x and earlier rendering and would be the default in LR 3 for any images that you've already edited in LR2.x or ACR5.x or before and have an XMP file with those settings.
Process 2 is the new LR3/ACR6 rendering and is the default for images that have not been imported or adjusted in an old LR or ACR.
You can change the Process version from 1 to 2 and back to 1 via Lightroom-Develop / Settings / Process Version.
To make a detailed visual comparison of Process 1 and Process 2 for a particular image, use the side-by-size Y mode in Develop, then use the <- to copy the After settings to the Before version, then go up to Settings / Process Version and set the opposite Process version, and compare the left-and-right images at 1:1 to see what is different. The rendering has finer detail in Process 2, there is no luminance noise-reduction, and the color-noise-reduction is better. Otherwise there isn't too much difference, except in how the postcrop vignetting works and maybe a couple other things.
Ahhhh! That explains a lot, thanks. I thought I was going mad, but I must have been forcing Lr3 into using the old process.
I will give it another spin. I wonder how many times I've installed and removed Lightroom in the last few years.