Funny, I've just been toying around the last few weeks with doing just this, for AmphetaOutlines, to cut down on the wodge of HTML it feeds the browser all at once. Marc Barrot presents Transclusion Breakthrough: The Endless Web Page. The post reads a bit like an advert for Amazing Live Seamonkeys!, but I think the enthusiasm is understandable:

This is the in-browser version of what Dave Winer and UserLand created for Radio's outliner.

This is
instant rendering, happening on the fly as you browse through the current page. It is totally recursive: try clicking on the 'endless web page' node that appears under my name in the demo page.
Now, I'd like to dissect and figure this out, and add it to my AmphetaOutlines hack so that I can stop loading 5MB (!!!) of HTML every time I reload the page. Beyond that, I can see some very cool applications involving live data navigation and outline rendering and... yeah. This is cool stuff.