Some people have asked what they should do to support IDNA2008 on Microsoft platforms.
We provide IdnToAscii() and related functions in the Windows SDK. That's available natively on Vista+, and through idndl.dll on earlier platforms. Idndl is shipped with IE 7, or through "Microsoft Internationalized Domain Names (IDN) Mitigation APIs" at the Microsoft Download Center.
For .Net, the IdnMapping class provides IDNA2003 conversion since .Net V2.
Obviously these APIs currently only support IDNA2003, however the interfaces won't change for IDNA2008 support. I don't know what mechanism will be used to update for IDNA2008 support, but applications should continue to use the exposed APIs for consistent support across the platform. That way if the system gets updated to IDNA2008, the application will be able to take advantage of the updated support.