Baz, I'm not sure how relevant this is to your problem, and I should mention up front I haven't managed to get my browser (Opera6) to display Adobe's svg test samples yet!

However, whilst I was trying to get this going, I was looking at the mime-type associations within Opera and noticed that extentions svg and svgz are associated with mime-type image/svg-xml rather than image/svg+xml as you are using in your header?

Maybe changing this would prompt the plug-in to handle the inbound datastream (or at least be given the chance to by the browser).

Unfortunately, the adobe install is a 'click'n'pray' affair giving no feedback on what it is doing leaving me with the task of trying to work this out what it did after-the-fact.

Good luck. I think what your doing is really interesting. You don't have a svg file of the mainland by any chance do you:)