Ah, interesting stuff. If no-one else beats me to it, I'll look into implementing Darren's text idea. I think it's clearly the best solution.
My general approach to icons is to eliminate as many as possible for the reasons outlined in the third numbered point of this usability assesment.
The icons that I've left generally convey some information over and above the text they are next to, e.g. the filetype (powepoint, pdf) of a resource file.
For some of the more decorative uses, such as the calendar icon that appears on the main calendar page, larger icons might be more appropriate such as the one I've attached below.