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)