The Mac OS X version of Graphviz currently has much better antialiasing of bitmaps; try running some of your dot files through it to see. It is independent of X11 and uses the native font and PDF machinery on that platform.
It also has a pretty interactive front end, you can tweak most command-line parameters or edit the file, and see the results re-rendered almost instantaneously.
The sources for the front end are BSD and are available from www.graphviz.org. Perhaps some brave soul can port it to GnuStep or something.
[BTW, I'm the author of the Mac OS X port.]
Glen Low, Pixelglow Software