Dudley,

Faces is one of the few features I don’t actually use, so hopefully someone else can chime in with more info. The only thing I can think to ask right now is, have you tried the command Photos > Detect Missing Faces on new photos? Clearly this is supposed to be happening automatically, but maybe it needs a little kick-start.

Have you tried trashing the preferences? You will have to reset a few prefs, but that’s often the easiest quick-fix for problems where Aperture appears to be “stuck”. The file is in ~/ Library/ Preferences/ com.apple.Aperture.plist and remember in Lion, to get to the ~/Library, hold down the option key while clicking the Go menu in the Finder.

Barring that, have you done any repairs on the database (holding command-option on launch).