Recently I've had some questions about resource loading/fallback in .Net and how it works.
Brian posted a ResourceManager Video Presentation on the BCL Team's WebLog. If you're interested in resource loading this might be helpful.
In general custom cultures work like other cultures for resource loading, so the normal resource loading behavior should remain the same.
Prior to Microsoft .Net Framework 2.0 some people were deriving their own cultures from CultureInfo and using those to help load resources. I've encountered several questions from people using that type of culture in 1.1. Since the derived cultures aren't quite the same as "real" .Net cultures, I'd recommend moving toward .Net 2.0 and real custom cultures when possible. The "real" .Net 2.0 custom cultures solve most of the problems encountered by users of derived CultureInfo objects.