DELETE a,b,c
FROM wp_posts a
WHERE a.post_type = 'revision'
LEFT JOIN wp_term_relationships b
ON (a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON (a.ID = c.post_id);

http://www.wprecipes.com/wordpress-tip-get-rid-of-unused-post-revisions

Just run the following query on your WordPress database, and all revisions (As well as meta associated with it) will be deleted from your database.

Get rid of unused post revisions

wordpress

MySQL