I've been photographing some of tourist route signs that use icons instead of numbers lately. I've also been playing with the trace feature in inkscape, which turns out to be surprisingly easy.

I made this one this evening:

http://wiki.openstreetmap.org/wiki/Image:Shield_australia_tourist_dugong.svg

Which I'm hoping can be rendered in future by people wanting localised highway shields.

I've spent a fair bit of time thinking about this and discussing it with Mike, and we think we have a suitable enhancement for osm2pgsql, we just need to tweak things a little to trigger some SQL queries after osm2pgsql has dealt with it's main task, and to null out the icon field if a road is updated so that new icons can be assigned if needed.

Hawke has also spent a fair bit of time updating the lookup page with suitable highway shields for the US:

http://wiki.openstreetmap.org/wiki/Custom_Highway_Shields#United_States_of_America