If you have the Mac OS X 10.3.9 update installed and have the debug menu enabled in Safari 1.3, take a look at the new menu option there: Use Transparent Window.
If you open a new, empty window or tab in Safari and then select that menu option, you get exactly what it sounds like.
My question is... WHY???? No seriously - what purpose does it serve? Here's what it looks like with just my dekstop image in the background:
It doesn't work with a browser window that has a page loaded in it, though. I won't sleep tonight until and unless I know what this feature was intended for, in a debugging environment or otherwise.