Composite Applications became defined when the Patterns & Practices group released the Composite UI Application Block - well they might have existed before then, but CAB made them more "mainstream". Acropolis was the supposed to be the next wave for building composite applications with WPF. The Acropolis team has made an announcement on the next phase of the Acropols project. In addition, the P&P team has provided some interim guidance for building composite applications with WPF - more to come, I'm sure.
I'm dissapointed that Acropolis won't be available anytime soon. I understand the need to "get it right" since I've been living with some of the pain with developing LOB applications with WPF for a few months now. I've been advocating building on top of the SmartClient Software Factory with the SCSFContrib project (with limited success with my current customer :(), so I'm glad to see the P&P team point to that as well....seems I might have guessed right ;).