Well, since Vista is available, it would only make sense that .NET 3.0 should also be available. It seems that it can be installed on Windows XP Service Pack 2 and Windows Server 2003 in addition to Windows Vista.

No compatibility for Windows 2000, but that might not be a big deal anymore.

Looks like all the .NET 3.0 stuff continues to use the Visual Studio 2005 platform for development so it might make sense to consider .NET 3.0 in your future SmartClient ambitions to get that next generation interface design today.

.NET 3.0 Framework Details Page