I just discovered another really fun wrinkle in this.

Let's say I want to have my entity resolver return a reference to my 
local copy of a DTD.  I write:

    source = xml.sax.InputSource()
    source.setPublicId(publicId)
    source.setSystemId(systemId)
    source.setCharacterStream(file(path_to_local_copy))
    return source

This will appear to work.

However, the parser will still silently fetch the DTD over the network!  
I needed to call source.setByteStream()--character streams are silently 
ignored.

I'd never have noticed this if I hadn't used strace on my process and 
noticed a slew of recvfrom() calls that shouldn't have been there.