I like this idea, for simple icons. Standard Unicode code points for symbols could be used to even allow screen readers to read them!

Another alternative for pages with simple icons is to use the Data URI scheme:

  <img src="data:image/png;base64,iVBORw0KGgANSUhE..."/>

This compresses very well with HTTP content encoding when the same icon is repeated in multiple places in the page: http://en.wikipedia.org/wiki/Data_URI_scheme