If you aren't playing with Windows Vista Beta 1 or Microsoft.Net v2 Custom Locales/Cultures, you can still get some support for new locales in XPSP2. A download for the following locales was posted today:
http://www.microsoft.com/downloads/details.aspx?FamilyID=3fa7cdd1-506b-4ca0-bd47-b338e337a527&displaylang=en
• Bosnian (Cyrillic, Bosnia and Herzegovina) • Filipino (Philippines) • Frisian (Netherlands) • Inuktitut (Latin, Canada) • Irish (Ireland) • Luxembourgish (Luxembourg) • Mapudungun (Chile) • Mohawk (Mohawk) • Nepali (Nepal) • Pashto (Afghanistan) • Romansh (Switzerland)