There are some stats I generate from the access logs of the
update check web server. They are available at
http://update.dokuwiki.org/stats/
The real values are probably much higher. The stat do not take multiple wikis behind the same IP into account. Or Wikis behind firewalls or with the update check disabled. Also Wikis not used daily wouldn't be counted correctly. The same goes for installs running versions older than rc2006-09-28.
I'd guess there are about 6000+ installations world wide. From looking at the weekend <-> weekday differences I'd estimate about 50% are corporate wikis.