CouchDB has released a simple authentication api but has no in built authentication mechanisms as of yet. The simplest and easiest way to do this is to use an http proxy for authentication. However this has limitations on how much you can restrict access on a per document basis. When CouchDB gets some more support for built-in authentication modules then it should be easier.
If you want to try your hand at coding an authentication module then you can check out the source for the javascript security_validation tests in this file:
http://svn.apache.org/repos/asf/couchdb/trunk/share/www/script/couch_tests.js
and the default_authentication_handler in this file here:
http://svn.apache.org/repos/asf/couchdb/trunk/src/couchdb/couch_httpd.erl
that would get you started anyway.