After you've tagged a bunch of pictures, you can view the list of all of the faces of people you know by clicking "Faces" over on the left-hand side. They appear in a grid, looking like a bunch of Polaroids all 'tacked up' on a corkboard.
However, iPhoto '09 will show you the first photo of each person you identified... even if they were way off in the distance, or blurry. Needless to say, this is NOT necessarily the best picture you have of them!
Fortunately, you can double-click one of the polaroids on the 'corkboard' to view a custom album of all photos that have that person identified in them. If you highlight one of the bigger/better/clearer photos, just pick 'make key photo' from the Event menu to make that the photo displayed for that person on the 'corkboard' display of all Faces.
[robg adds: As revealed by Phil Schiller during the Macworld Expo keynote, there's another little trick to use when browsing faces -- just hold down the Option key while looking at a face on the corkboard, and you'll see the 'zoomed out' version of each photo as you move your mouse across each collection of faces. It'd be nice if there were a way to set the key photo from this view, but if there is, I haven't found it yet.]

