I don't seem to be able to fax from Illustrator 10 using the built-in fax support in Panther. But when I save my files in PDF-compatible Illustrator format, I can open my documents with Preview and fax them from there ... a simple hint, but one that has saved me some time!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031202121515113