I did respond, although perhaps too humorously. My suggestion is
that you use the URNs of Frequently used icons instead of URLs.
Some clients might even have built into them copies of resources that
are really frequently used (like common inline images).
Others might retrieve them every time and just cache them for a short time.
You might argue that 'URNs aren't here yet', but defining a standard
for URNs really isn't harder than defining a standard for /inline/etc.
images.