artist-mode to create maybe, but then how do you edit? Concepts such as "make this box bigger" or even "move this box over here" get kind of difficult when it's all just pixels or all just ascii chars.
Graphviz is pretty cool though. But saying that I tend to have to spend too much time tweaking it just so that all the labels don't overlap.
For more flexible diagram work try the Artist-mode/ditaa combination (http://www.emacswiki.org/cgi-bin/wiki/ArtistMode, http://ditaa.sourceforge.net)
Or course once you get really serious you'd use Graphviz http://www.graphviz.org
Diagrams in text = version controlled = win!