But the $64,000 question (actually, morel ike a $200 question) is not what ACR 6 will be like but whether it will be backwards compatible with CS4 (not that I expect to find out until April 12).
New versions of ACR have always had improvements that cannot be implemented by previous versions.
Just like PS adds items like say layer styles that cannot be utilized by versions of PS that did not have those features.
And this is the way of software improvement that is near universal when it comes to files made with new features.
Why would you need backwards compatibility unless someone else is processing images for you in a more recent edition of ACR, which you then wanted to re-edit in an older version?
An unlikely situation. If you have the new version, why bother with the older less able variant.