Ham radio operators trade what are called QSL cards. In the international "Q" code, this is an acknowledgement of receipt of a message. These QSL cards are then collected and displayed. Just as with avatars, people devote time and effort into their design.
It would be cool to have Cache Site Location/Located cards to collect. Inside the cache, there would be site specific cards for finders to take. Finders could leave their Cache Site Located cards for cache owners to collect during cache maintenance.
If the physical aspect is too costly (or inappropriate for microcaches), this could also be done electronically when caches are logged. You could then have a digital scrapbook of CSL showing off your caches found as well as one with CSLs that include the comments of those geocachers who have visited your caches.
In the ham radio world, designing your QSL cards was/is an essential task. As such, this same topic came up in the ham radio forum a while back. See "QSL cards for caches" last updated Sept 2002 but started in February 2002.
http://ubbx.Groundspeak.com/6/ubb.x?a=tpc&s=5726007311&f=4216058331&m=9766024841
geo-jedi
Team Geo-Jedi, Searchers 4 Ground Truth