I tried copying the Camera Raw.8bi from C:\Program Files\Common Files\Adobe\Plug-Ins\CS6\File Formats\Camera Raw.8bi to C:\Program Files\Common Files\Adobe\Plug-Ins\CS5\File Formats\Camera Raw.8bi (after renaming the ACR 6.7rc Camera Raw.8bi to Camera Raw.old8bi(in case I needed to go back, and I did!) and IT DOESN"T WORK in CS5!!!
Looking at the CS5 properties, it reports ACR as 7.0, BUT, you CANNOT OPEN raw image files directly in CS5 Bridge, nor can files be moved from LR4 to CS5 with the 7.0 plugin installed.
Gotta wait for shipping CS6 to get the full benefit of LR4 to CS6 compatibility.