If you have the 'opacuserlogin' pref set to 'Don't allow' and 'TagsEnabled' set to 'Allow' then the form to add tags appears (where you've turned it on) but tags entered in it are not added. The tags box shouldn't appear if 'opacuserlogin' is turned off - or tagging shouldn't be linked to usernames - either way this is a bug that needs addressing.
Created attachment 2899 [details] [review] Proposed fix
Pushed please test
This bug is mentioned in: Fix for bug 5451, can add tags with userlogin turned off http://lists.koha-community.org/pipermail/koha-patches/2010-December/013458.html Fix for bug 5451, can add tags with userlogin turned off http://lists.koha-community.org/pipermail/koha-patches/2010-December/013453.html