I confirm this bug.
In my configuration, I use group names with a capital letter as the first letter: "Profs".
It used to work fine. Since the update with "dokuwiki-rc2009-12-02.tgz", it does not work any more. The members of group "Profs" are not recognized any more by the ACL line mentioning @Profs.
If I change the group name "@Profs" to "@profs" in the "acl.auth.php" file, then it works fine.
I also noticed that if I add a rule for a new group "Hello" through the administration interface, then it generates a "@hello" group name in the "acl.auth.php" file, with a lowercase initial letter.
This sounds consistent with the remark of Jonas above.