The new package is subsite aware (and instance aware), it also has private address books (the way that it works is similar to the way that there are shared calendars in the calendars package). The new contacts package removes all attribute management features into a new package called ams. its not really pluggable into the old model since its a totally different backend (that's WAY more efficient and easier to maintain. There shouldn't be scalablility issues with the new contacts package (as there are with the one in CVS now). I am working on getting it into a state that can be uploaded to CVS and hope to have it done soon, but since its a side project for me and there are constantly things coming up i can't give you a timeline that will mean anything.
