A really good(*) developer mailing list maintained by the Omni Group in Seattle. http://www.omnigroup.com/community/developer/mailinglists/macosx-dev
*really good is the content and the contributors, really bad is the mailing list software… it’s been having some “issues” lately.
The Omni list is a good place to ask questions about using unsupported API and the like - you’re more likely to get a useful answer there than on the official Apple lists.