In case you've been living under a rock, or you don't spend all day hitting Refresh on Microsoft sites waiting for new stuff to come out, Acropolis is a new framework currently in CTP (Community Technology Preview) from Microsoft. Basically Acropolis is a framework providing support for composite application development using Visual Studio "Orcas", WPF (Windows Presentation Foundation) and the .NET Framework v3.5 (currently in Beta 1).
To get Acropolis, you need to download the 5.7GB disk image for Orcas and install that. Once you've managed to install that whopper, you then download the Acropolis CTP. If you're feeling lucky, also go ahead and download the Silverlight, Astoria, and Jasper CTPs. Hell, if you're going to slap a 5.7GB beta on your disk, you might as well drop all the goodies from Microsoft on the same machine (or virtual machine).
