afaik, ACR plugin is not 'used' by lightroom itself.
the reason for upgrading (elements/photoshop) to acr 3.7 is it has been updated to recognised lightroom info. so when you later open the same raw file in cs2/elements, it can use that info.
in the help file (under export) , it states
"Note: Versions earlier than Adobe Camera Raw 3.7 will not recognize some of the develop adjustments made in Lightroom."
as an aside, i can see that under calibration in develop, you can see the ACR 'profile' used, but i guess lightroom shares the profile data... for me with a 20D this shows 2.4, as i guess this was the last time it was updated.