Well, I've heard back from the PowerPoint team, and they have confirmed that "Insert from scanner" has been removed from PowerPoint 2007.

Mark explains:

"This was a difficult call, but I think for the best of the product overall.  The feature supported a limited number of scanners and the camera support had not been updated for some time."

While this may mean a longer process to add scanned images into PowerPoint, hopefully the extra features in the 2007 release will make that worthwhile. And maybe the improved graphics will reduce the need for scanning in some cases.