iPhoto '09's new Faces feature works well, but sometimes misses faces in photos. If that happens to you, you can find the missing faces by clicking the Name button, then clicking the Add Missing Face button. When you do, iPhoto places a box around the face, which you the drag to resize to cover the entire face of the person of interest.
By default, the drag-resizing occurs from the center of the box outwards. To me, this feels quite strange an unnatural, and I was having trouble quickly drawing boxes of the right size and shape.
Thankfully, there's an easy workaround. If you hold down the Option key before dragging, the box will be anchored to the corner opposite the one you're dragging--drag the top-left corner, for instance, and the bottom-right corner will remain fixed in place as you drag. Move to another corner, and its partner will then remain anchored as you resize.
I find this corner-based resizing more natural and much faster to use. Unfortunately, I haven't yet found a way to make it the default behavior, so for now, you'll have to rely on the Option key if you prefer this method.