MAYBE this will work (haven't tried it):
SELECT editorcount as 'Number of unique editors', COUNT(rev_page) as 'Number of articles with that many unique editors' FROM ( SELECT rev_page, COUNT(DISTINCT rev_user_text) as editorcount FROM revisions GROUP BY rev_page ) AS editorcounts GROUP BY editorcount ORDER BY 'Number of unique editors';
I've just tested this, and it works perfectly, except that "revisions" should have been "revision". :-)
Timwi