Just wanted to share an observation that I had with my own Aperture library regarding Faces. I have installed the latest greatest 3.0.2 update and while it fixes a lot of things and appears to be much more stable - even with my limited 5 or 6 hours of use - the one thing it does not seem to help with is the "missing" face issue.
Like a lot of you out there some of the face extracts from my photos are blank. When I look at the full view of that photo it is fine and the face box is perfect - I have even tried deleting the "face box" and redrawing / renaming it to no avail. Getting to my observation, which is by no means comprehensive or scientific, I can say that in my library every single solitary photo that has this issue is a JPG - not only are the trouble causing images JPGs they are all associated with RAW+JPG "dual masters" as I do not have any stand-alone JPGs in my library.
When I have time I will try a more scientific set of experiments to see if I can find the root root root issue but it is very strange that this factor seems to hold true for my entire library.