The short of it is that the project in its current form (as CTP's) will no longer be versioned (but will remain available.) However, the goals, concepts, and lessons learned from Acropolis are being transitioned to the Patterns & Practices team for long-term direction and development of guidance:
"We are very excited to announce that we are going to be working closely with the Microsoft Patterns & Practices team to provide guidance (samples, applications blocks, patterns and so on) for building composite client applications for .NET Framework 3.5 and Visual Studio 2008."
The post includes a summary of guidance for customers who have been considering how to implement composite applications:
"If you have evaluated Acropolis and are unsure whether to adopt it for your project, or to use the existing CAB, or to wait for the new guidance, our guidance for this situation remains the same - if you are building a Windows Forms LOB composite client (with maybe rich islands of WPF content) you should carefully evaluate the current CAB release. If you are specifically interesting in building composite applications on .NET 3.5, please get involved with the Patterns & Practices project and help us to deliver a guidance package that meets your requirements."
In a related announcement, Glenn Block of the P&P team has announced plans around the WPF Composite Client. It will be a new set of libraries and guidance, not a new version of the existing CAB, to be delivered in several smaller releases so customer feedback can be incorporated into subsequent iterations.