Update (2006-01-12): Version 1.4 released, which is updated to work with WordPress 2.0. It also provides better error messages if it can’t authenticate the user. You can download a zip from Owen Winkler’s site.
Update (2008-04-16): Changes in WordPress 2.5 are causing problems with this plugin. I’ve released an updated plugin that is compatible with the upcoming WordPress 2.5.1.