It was possible to get a preview of AutoCAD Plant 3D at AU 2007. A few more details in this AECCafé article with indication that AutoCAD Plant 3D might be released in 2008. Not a big surprise that it will be AutoCAD based as AutoCAD P&ID is AutoCAD based. It has probably been a lot of thoughts behind the selection as Revit was the other platform candidate Autodesk has. It shows a firm commitment for AutoCAD and those with AutoCAD Architecture might feel less threatened by Revit.
Will it be AutoCAD Plant 3D 2009 then? If Revit is the BIM platform, is AutoCAD the PIM platform (Plant Information Modeling)? Both can at least end up in NavisWorks.