SVG graphics support was suggested on wikipedia-l. I like the idea; that's why I tried it for Nupedia once, but... :-( Proposal: [[svg:foobar]] is just plain-text editable, but displays as an image; A link to [[svg:foobar]] is displayed as an image as well. Optional, after saving an edit to [[svg:foobar]], an image (foobar.png) could be created via svg2png (never tried that, though). The image would then be displayed instead of the SVG, and "real" SVG display could be turned on in the user options (until most browsers support native SVG).
That's pretty much what I had in mind as well, but SVG isn't mature enough yet to think about. For now, people can upload the "source file" for a drawing in SVG, as well as uploading a PNG version, and just link to the PNG in the article.