SharePoint MOSS 2007 - Custom Master Page causes 401 UNAUTHORIZED for anonymous users
To do that, go to the root of your site and login as your site collection administrator. From the Site Admin menu, select "Site Content and Structure". Browse to the Master Page Gallery, Select the Master Page you were working on and select "Approve". Go though the approval steps, then try hitting the site again as an anonymous user. That should fix the problem.
I hear that this work around isnt available for the base Sharepoint 3.0 Server product (without the MOSS component), so you'll have to do more searching on google for another work around there >_>