I've known about, tracked and been briefed on Acropolis for a year now. I'm obviously interested given both my Smart Client preferences as well as our extensive use of CAB + SCBAT in our composite Enterprise type of client. Microsoft finally announced Acropolis at TechEd and released a CTP and Docs. We have been greatly concerned that CAB development has pretty much stopped (now everyone sees why) and how to possibly take the code base into WPF if we wanted to (it would make sense to use one API set WPF/WPF/e/Silverlight and target both the web & smart client), among other things like ObjectBuilder's performance problems, missing services and much more. Acropolis does a much better job than CAB in abstracting out pieces of itself as well as technologies like WPF. It's core is neutral. It also has a lot of important services like roles that we have had to build out. I obviously will have much more to say but I don't have time right now :). I do want to point to Brad Abrams excellent collection of links/answers to common questions (you can add me now!) and the CTP (above). I do need time to evaluate this so I am not making any judgements yet.
Also, Kathy Kam, who I know from the CLR team is the PM for this so check out her blog as well.
Now you can go to the "Source" and get David Hill's Introducing Acropolis