Cyntact

Cyntact is a tweak that alters the iPhone contact list to display a thumbnail of the photos you've already assigned to each contact. The images, which are just the right size for quick scanning, appear directly to the left of each name in your phone directory. The app costs $1. For that, maybe you can shave a few microseconds off your search for the appropriate contact (assuming you can identify a face faster than reading a name). Worth it? That's really your call. The app works as advertised and doesn't slow down scrolling.

As of August 2010, the app now supports high res pics on the iPhone 4.

Note: the screenshot here is obviously a partial shot provided directly by the developer. No way I'm posting my contact list here.