A couple of people have asked me to write up some more in-depth material on interfaces. I like that idea, and will do it at some point. Until then, here are a few links that I came across that cover the ins and outs of interfaces.

'Interfaces: Off the Beaten Track' by Malcolm Groves
'Code Reuse Through Interfaces' by Rob Collins
Joanna Carter's articles 'Hairier Parrots' and 'The All Seeing Eye'

If you know of other URLs, comment here and I'll roll them up to this post.