Take for example http://www.dracos.co.uk/play/locating-postboxes/ and look at http://www.flickr.com/groups/postboxbypostcode/
There are many excellent postbox groups already but they all have, to my mind one problem - no structure enabling anyone to find anything. This group will only contain boxes which have their postcode in the title or tags, enabling easy searching.
if you can't find it, or reference it, it's useless.
I therefore propose to tag the postbox pics with 'ukpostbox:XXX_YYY where XXX is the 1st part of the postcode and YYY is the box ID
This means that 1) things like locating-postboxes could bring up a set of pics of the boxes, flickr could automatically link to posting times (uhm but that assumes the royal mail has an API ha ha ha ha ha). If people have followed the Tagging guidelines on openstreetmap, then you can even link them directly together.
Maybe sir Tims vision wasn't so bad after all :-)