Longhorn finally has a name, so now I have to remember to type Windows Vista :-)
I'm a bit busy, but I wanted to write a quick post to mention that Windows Vista Beta 1 has Custom Locales. These are similar to .Net Framework V2.0 Custom Cultures, 'cept that we call them locales in windows and cultures in the framework.
Basically to make a custom locale in Windows Vista, follow the .Net Framework instructions. When you call CultureAndRegionInfoBuilder.Register() the culture is registered and immediately available to both native Windows and managed .Net applications! If you want you can even go to the "Regional and Language Options" control panel (from the control panel menu or start->run intl.cpl) and pick your new custom locale.
You'll notice that custom locale data is available in applications such as Outlook. For some applications I'd imagine that if you exceed the application's expectations too much you might run into interesting behavior. I made a sample Hawaiian locale that works nicely and I haven't found problems, but I expect you'll have problems if you try to get applications to recognize your custom Klingon PUA font seamlessly :)
If you do play with custom locales I'd be interested in hearing about your experiences, particularly if you run into any issues. (Klingon pIqaD isn't supported though, I'd like to hear about "real" locales :-) )